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She is first encountered by Karl "Helo" Agathon when he is stranded on Caprica. Initially assigned to seduce him by impersonating Lieutenant Sharon "Boomer" Valerii, as part of a Cylon cross-breeding experiment, she turns against her people and helps him escape the planet. Pregnant with Helo's child, Helo is shocked by her true nature but ultimately accepts her lovingly as the mother of his child.
Once on Galactica, she is imprisoned and long seen as a mere object. She frequently provides intelligence, but is severely mistreated by an interrogator from Pegasus who beats and sexually assaults her. Over time, she earns Admiral Adama's trust by helping the Colonial Fleet escape peril several times, and she is eventually appointed as a Colonial Officer and marries Karl Agathon. Agathon gives birth on Galactica, but her daughter, Hera, is subsequently hidden from her, because President Roslin considers the baby a threat to the Fleet. Agathon believes Hera to be dead for more than a year, but after learning the truth about her child during a confrontation with Valerii, she is able to regain her daughter from the Cylons with the help of Caprica-Six.
She later has to deal with Boomer returning, beating her terribly, seducing Helo and kidnapping Hera. This causes her to fall into a depression until she finally rescues Hera during a final battle that destroys the Cylons. Also she gets some revenge for what Boomer did by being the one to permanently kill her. Afterwards, she gets to live out a peaceful existence on new Earth with her husband and daughter.
Cylon cross-breeding experiment on Caprica
Sharon is a Number Eight model first seen on Caprica shortly after the Fall of the Twelve Colonies and the Cylons decide to use her in an elaborate experiment focused on Karl "Helo" Agathon who is stranded on the planet after an emergency landing in Raptor 314. She is an identical copy of the same model as Helo's friend and co-pilot, the Cylon sleeper agent Lieutenant Sharon Valerii, and the Cylons hope to explore the possibilities of Cylon-human procreation by getting Helo to fall in love with her (TRS: "The Farm"). Sharon appears to Helo to rescue him after he is captured by a Six and several Cylon Centurions. Unaware that Cylons have developed human models, Helo believes her to be Sharon Valerii from Galactica, returning to rescue him (TRS: "33"). Following this staged rescue, she leads Helo to a Raptor, now surrounded by Cylon Centurions, and convinces him that they have no direct way off the planet (TRS: "Water"). Sharon is able to impersonate Lieutenant Valerii convincingly, since she has accessed Valerii's memories at least up until the point of Helo being left behind on Caprica (TRS: "Flesh and Bone", "Scattered", "Home, Part II", "Flight of the Phoenix", "Scar").
After receiving a feigned Colonial signal on a radio receiver they carry, Sharon leads Helo to a city where they find a fully-equipped fallout shelter in which two people can live in reasonable security, hidden from above-ground Cylon operations, and with sufficient supplies to last a considerable period of time (TRS: "Act of Contrition"). The purpose in establishing this retreat is to elicit an emotional response in Helo towards his Cylon companion. When this fails and Helo continues to seek a way off Caprica, the Cylons lead him to believe that Sharon has been captured, concluding that if Helo does not seek to rescue her, the experiment has failed and he must be killed (TRS: "You Can't Go Home Again").
Helo does undertake a successful rescue attempt and shortly after, the two of them have sex for the first time in a nearby forest. Helo had always been fond of the original Valerii on Galactica, but his affection for her was stilled by her relationship with Chief Galen Tyrol (TRS: "Six Degrees of Separation"). Pleased with the progress, the Cylons set up a new hideout, a cabin in the woods, and Sharon is instructed to lead him there and have him stay with her – or kill him, if he attempts to leave (TRS: "Flesh and Bone"). But she begins to question the Cylons' overall goal. Realizing that she has herself fallen in love with Helo, Sharon disobeys her instructions and leads Helo to Delphi, where they hope to steal a vehicle and get off the planet (TRS: "Tigh Me Up, Tigh Me Down"). On their way to Delphi, she shows signs of being pregnant, succumbing to a bout of morning sickness (TRS: "The Hand of God") and developing a ravenous appetite (TRS: "Colonial Day").
Revelation of her true identity and arrival at the Fleet
On reaching Delphi, Sharon and Helo break into the Cylon facilities to gain access to the spaceport. But as Helo encounters yet another Number Eight model, he draws the initial conclusion that Sharon is a human clone created by the Cylons and goes on the run alone (TRS: "Colonial Day"). When Sharon catches up with him, she tried to calm him down and eventually challenges him to shoot her, after he tells her to stay away (TRS: "Kobol's Last Gleaming, Part I"). Helo can only bring himself to wound her, and he takes Sharon with him to use her in some way to get off Caprica. While Helo keeps her at gunpoint, Sharon leads him to the Delphi Museum. Waiting out a storm in a nearby ruined building, she explains to him that she is “Sharon”, not a mere clone or a copy, and that she is as real as Sharon Valerii on Galactica. She also tells him that her love for him is real and that she is pregnant with his child.
When Sharon and Helo come across Lieutenant Kara Thrace who has landed near the museum to retrieve the Arrow of Apollo, Thrace realizes on-sight that Sharon is a Cylon (having left Valerii behind on Galactica) and she attempts to shoot her. Helo stops Thrace and reveals to her that Sharon is pregnant with his child (TRS: "Kobol's Last Gleaming, Part II"). Thrace also wrongly concludes that she must be a Cylon copy of the real Valerii on Galactica; Sharon's attempts to explain her identity to her are unsuccessful. While Thrace and Helo argue over her, Sharon escapes in Thrace's Cylon Raider to save herself and the life of her unborn child (TRS: "Scattered").
Several days later, Sharon returns to Helo who is now with a human resistance movement that has formed on Caprica. She offers her help in finding Thrace who was abducted during a Cylon ambush (TRS: "The Farm"). Thrace manages to escape to the outside of the hospital where she was imprisoned and Sharon arrives at the rescue scene in a stolen Cylon Heavy Raider, destroying several Cylon Centurions that attack the Resistance rescue party. Following the successful rescue, Sharon accompanies Helo and Thrace on their way back to the human Fleet in the Heavy Raider.
When they arrive at the Fleet and Sharon walks aboard the Astral Queen, Captain Lee Adama reacts violently. The son of Commander Adama who was shot by the Cylon "sleeper" agent Valerii grabs her and places his gun to her head. Helo draws his sidearm against Adama, forcing a standoff. President Laura Roslin urges both men to withdraw their weapons, but then immediately orders Sharon to be ejected out an airlock. Sharon tells Roslin she knows the location of the Tomb of Athena, an ancient place Roslin is trying to find, and she reconsiders, placing Sharon in the brig. Roslin later explains that she suspects Sharon is willing to cooperate, because she wants to protect Helo and her unborn child, something Roslin interprets as a mothering instinct.
Guide on Kobol
Sharon accompanies Roslin's party to Kobol. She recites specific passages of the Tomb of Athena in the scriptures of the Sacred Scrolls and plots the group's path on the planet. On their way they are ambushed by Cylon Centurions who open fire. As the others take cover and return fire, Sharon vaults away, with Captain Adama in pursuit, believing she is trying to escape. Sharon picks up a grenade launcher lying ahead, and as Adama thinks she is about to shoot him, she aims for the remaining Centurion and destroys it (TRS: "Home, Part I"). At camp, Adama voices suspicions about Helo who still loves Sharon, despite his awareness of her true nature as a Cylon. During the trek, Sharon casually tells Helo that their child is a girl.
Shortly after, Commander Adama’s search party arrives at Roslin's camp. When Adama sees Sharon, who looks exactly like the Cylon who shot him, he looks her over while remembering events that had happened and tries to choke her to death shortly thereafter. However, chest pain from his fresh wound forces him to release her. After the attempted strangulation, Sharon enigmatically references what Adama said over Sharon Valerii’s dead body, saying “and you asked why” aloud.
Later, one of Tom Zarek's followers, Meier, tries to convince Sharon to help him kill both Adamas, as part of a plan to gain control of the Fleet. Sharon initially believed that Sharon Valerii was being held in the brig, but Meier informs her that she was killed. She expresses her outrage to Helo that Cally received only a minor punishment for this murder. Sharon deduces that the Colonials do not see humanoid Cylons as people and that she must take matters into her own hands to prove her trustworthiness. Sharon appears to take up Meier's proposal to kill both Commander Adama and Captain Adama, but as Meier draws his weapon, she kills him instead, saving the lives of both Adamas. She announces to Commander Adama that she is not the same Cylon who shot him, and that she has no hidden protocols waiting to be activated; she makes her own choices. She then surrenders her weapon to Adama and successfully leads them to the Tomb of Athena.
Cooperation with the human Fleet
Sharon is brought aboard Galactica and imprisoned in a new reinforced cell that was originally designed to incarcerate Sharon Valerii. The virtual Number Six tells Dr. Gaius Baltar that Sharon's baby will be born in that cell and that the child belongs to Baltar and her (TRS: "Home, Part II").
During her imprisonment, Sharon nearly has a miscarriage, and is brought to sickbay where Dr. Cottle succeeds in saving the fetus. A Number Three model who poses as the reporter D'anna Biers in the Fleet News Service, stumbles upon Sharon while filming a documentary on board Galactica and she manages to send footage of Sharon back to Caprica. The Cylons are surprised, yet overjoyed, that Sharon is still alive. They are concerned that her hybrid child survives; saying that it must be protected at all costs (TRS: "Final Cut").
When Galactica experiences mysterious computer failures and system malfunctions from a mutated version of a Cylon virus, Commander Adama orders Helo to show the incarcerated Sharon the strange Cylon code. Sharon identifies it as a virulent logic bomb that will take control of the ship and kill off the crew without her help. Reluctantly, on advice from President Roslin, Adama brings Sharon to CIC, where she cuts her arm open and connects herself to a fiber optic data line to communicate with Galactica's mainframe computer. On accessing the computer, she takes into her a portion of the logic bomb code, and then instructs Lieutenant Gaeta to wipe the system's hard drives to erase all Cylon virus traces. As the crew restarts the ship’s computers, a massive Cylon fighter force jumps into range, leaving Galactica defenseless. Sharon makes adjustments to the code and transmits the altered virus from Galactica to the Cylons, disabling the entire fleet of Cylon fighters allowing Galactica's fighters to easily destroy them (TRS: "Flight of the Phoenix").
Interrogation and abuse
After Galactica reunites with battlestar Pegasus, Admiral Helena Cain sends Lieutenant Alastair Thorne to inspect the incarcerated Sharon. He beats and sexually assaults her with the help of several of his guards. Helo and Chief Tyrol find out about Thorne and what was done to his previous prisoner, and they arrive in time to stop him, but Tyrol accidentally kills Thorne in the process. Both Tyrol and Helo are arrested by the Pegasus guards and taken to Admiral Cain's Battlestar, where Cain hastily convenes courts-martial for them, sentencing both to death (TRS: "Resurrection Ship, Part I").
Sharon's attempted rape results in a hairline fracture in one of her ribs and minor bruising. Commander Adama who has reached a détente with Admiral Cain over Helo and Tyrol's execution personally apologizes to Sharon over the incident; this is also the first time that Adama refers to Sharon as "her," instead of "it." Later, Adama asks to speak with her in private, questioning her why the Cylons hate humanity. She responds by reminding him of a speech he gave before the attacks, in which he suggested that humanity is a flawed creation, and she implies that therefore humanity might not deserve to survive. This conversation causes Adama to decide not to go through with assassinating Admiral Cain. Cain similarly decides not to kill Adama but for an unknown reason. Sharon is later reunited with Helo and Tyrol (who she pointedly ignores) when the two are released after Cain's death, as Adama resumes full command of the Fleet (TRS: "Resurrection Ship, Part II").
Weeks later, President Roslin, on her deathbed, orders that Sharon's fetus be aborted, fearing the baby might be dangerous for the Fleet. On hearing this news, Sharon is enraged, having cooperated fully to aid the Fleet to save herself and her child, and to show that not all Cylons are dangerous. Marines are sent to take her to sickbay to forcibly perform the abortion procedure. At the last minute, Dr. Baltar learns that the fetal blood of Sharon's baby has the ability to destroy cancer cells. He takes a small blood sample and injects it into the dying Roslin; moments later, her cancer disappears. The abortion procedure is cancelled (TRS: "Epiphanies").
Despite her mistreatment, Sharon continues to support the Fleet and provides intelligence. She helps Captain Thrace to understand the behavior of Cylon Raiders (TRS: "Scar"), and frequently assists Admiral Adama to plot the Fleet's course in order to avoid Cylon confrontation. Colonel Tigh remains extremely skeptical of her increased involvement. Terrorists take hostages on Cloud Nine and demand her turned over in exchange for their lives. Adama questions Sharon if she'd tell him the names of the Cylons in the Fleet, but when she refuses, it just seems to reaffirm her value to him and he pretends to kill her himself and then turn over her body, in reality the Boomer copy of Sharon's dead body. Unfortunately the terrorists figure out the deception quickly and one hostage is killed, but the distraction allows marines time and a distraction to resolve the situation. (TRS: "Sacrifice").
Birth of her daughter Hera
When Sharon begins to bleed due to a detached placenta, Dr. Cottle performs an emergency birth procedure to save both her and the baby's life. Sharon and Helo name the baby girl Hera. After her parents believe Hera is out of danger, the baby apparently dies due to her underdeveloped lungs and Dr. Cottle's failure to intubate her in time. Sharon, suspecting her child was killed deliberately, nearly strangles Dr. Cottle before being subdued, and she subsequently falls into a deep depression.
However, unbeknownst to Sharon and Helo, Hera is still alive. President Roslin who still considers the child a danger for the Fleet had Sharon's baby switched with the body of a dead child and, with the help of Dr. Cottle and her aide Tory Foster, places Hera in the care of Maya, a foster mother. Maya is told a cover story that disguises Hera's true identity (TRS: "Downloaded"). Admiral Adama is not informed of Roslin's plan (TRS: "The Eye of Jupiter").
Sharon's depression over losing her child worsens and Helo convinces her to aid in the rescue of Samuel Anders and other survivors on Caprica, in part to try and draw her out of her depression. Sharon's knowledge of Cylon FTL technology makes the rescue possible. She acts as a conduit between the Colonial Raptors and the far more advanced FTL computer salvaged from the Cylon Heavy Raider she once brought back to Galactica (TRS: "Lay Down Your Burdens, Part I").
On Caprica, Captain Thrace demonstrates a new level of trust for Sharon by handing her a gun during a firefight with ambushing Cylon Centurions. Later, Sharon is as perplexed as everyone else when the Cylons disengage and apparently leave the planet. She is aware, however, that Cavil, a man who approaches the group and loudly proclaims the end of the occupation to be a miracle, is a Cylon. On Galactica, Cavil is exposed by Chief Tyrol, and Admiral Adama sends Sharon back to the brig. When asked by a distraught Helo why she did not mention that Cavil is Cylon, she replies "They killed my baby. Do you think I care about you, or us, or whether or not Adama trusts me anymore?" Sharon withdraws further, shutting Helo out completely (TRS: "Lay Down Your Burdens, Part II").
Officer of the Colonial Fleet
More than a year later, and four months after Galactica and Pegasus's retreat from New Caprica, Sharon Agathon is now married to Helo. In the year that has passed, Admiral Adama has taken Sharon into his confidence and she seems to have become his advisor of sorts having apparently fully gained his trust. The two periodically sit down to tea; her cell now has a couch, a table, and other amenities. She tells him that she has come to forgive herself for her actions against her race and losing her daughter and that he must forgive himself for leaving the people behind on New Caprica if humanity and the Fleet are to survive. (TRS: "Occupation"). Thanks to her giving him his confidence back, Adama decides to forgive himself for leaving most of the Fleet behind and go ahead with the rescue mission. As part of the plan Adama commissions Agathon into the Colonial Fleet as a lieutenant (junior grade). When she asks how he knows if she won't betray him, he simply replies he doesn't, but "that's what trust is." On her first mission, Agathon leads an insertion force that will serve as a liaison between the New Caprica Resistance and Galactica during the extraction of the population. She commands two Raptors in an intra-atmosphere jump and links up with Samuel Anders and his squad at the Breeders Canyon. Due to an ambush she is nearly killed, but is saved since her Marines anticipated a possible ambush and were in position to deal with the threat (TRS: "Precipice", "Exodus, Part I").
On the following day, Agathon covertly enters the Cylon detention center to recover the launch keys for the grounded Colonial ships, bypassing human and Centurion guards that cannot distinguish Agathon from other Eights. After interfacing with a data terminal and obtaining the keys, a Number Three enters the room and challenges her loyalty. The Three tells her that Hera is still alive, but Agathon rejects the notion saying that Adama wouldn't lie to her (he didn't: he didn't know the truth), shoots the Three in each leg to slow her eventual report back to other Cylons. When she hands over the launch keys to Chief Tyrol, she asks him if he had actually seen Hera's ashes, which he affirms (TRS: "Exodus, Part I").
Following the Battle of New Caprica, the Agathons and many other pilots eat in the mess hall. After Racetrack addresses Agathon as "Boomer," Helo asks the crew for suggestions for a more appropriate callsign. A number of humorous and affectionate responses, including "Chrome-dome" and "The Raptor Adaptor" arise, reflecting Agathon's grown trust and comfort amongst the human crew, but Helo opts for Hot Dog's suggestion, "Athena" (TRS: "Torn").
Agathon undertakes a solo recon mission through a high radiation cluster to look for a planet with suitable material to replenish the Fleet's lost food supplies. Agathon's Cylon physiology withstands radiation levels that would have killed a human, although not without some discomfort. The successful mission confirms the foodstuffs on a planet with algae and gives needed data to safely attempt the guide the Fleet (TRS: "The Passage").
Reunion with her child
During a stand-off at the Algae planet, a Cylon delegation boards Galactica to negotiate for the Eye of Jupiter. Agathon, near the conference area, instantly identifies the Eight from the baseship delegation as Boomer. Valerii tells Agathon that her daughter, Hera, is still alive and also notes President Roslin's involvement in her daughter's disappearance. After confirming the matter with Roslin, Admiral Adama informs Agathon that Hera's death was indeed fabricated without his knowledge. Agathon angrily demands to see her child (TRS: "The Eye of Jupiter").
Agathon convinces Helo that the only way to rescue Hera is for her to download to the Cylon fleet and infiltrate the basestar where Hera is kept. Reluctantly, Helo agrees to shoot her and Agathon is subsequently downloaded into a new body and resurrected on the nearby resurrection ship where she is greeted by Caprica-Six upon rebirth. Agathon pretends that she wants to stay with the Cylons and Caprica-Six leads her to Hera as Caprica-Six is apparently at least somewhat sympathetic to her. Her daughter is ill and apparently rejecting Valerii who has been taking care of the child on board the Cylon vessel. Following Agathon's demand that Hera needs to see a human doctor, Valerii threatens to kill Hera, but Caprica-Six intervenes and kills Valerii instead. Accompanied by Caprica-Six, Agathon and her daughter leave the basetar in a captured Raptor and return to Galactica (TRS: "Rapture").
Strange visions and the search for Earth
Several weeks later Agathon is assigned to fly Major Adama and Gaius Baltar's new defense attorney Romo Lampkin to Colonial One to retrieve documents needed for the up-coming trial. Before take off, a remote-detonation bomb attached to the Raptor is fortuitously discovered. Cally Tyrol brings up Agathon's name and indirectly asserts she is the the unknown bomber. When confronted by Agathon, Tyrol denies making such an accusation, though Agathon goes on to note how she did not accuse her despite equal circumstantial evidence implicating her. Agathon points out Tyrol's deep hatred for all Cylons and hands over her sidearm, challenging Tyrol to shoot her. When she fails to pull the trigger, Agathon grabs the gun back and threatens to kill Tyrol if it is discovered she was responsible for the bomb on her ship. It is later revealed that the explosive was the work of fellow Galactica officer Captain Kelly and was designed to kill Lampkin (TRS: "The Son Also Rises"). During the time of Baltar's trial, Agathon is experiencing mysterious dreams of President Roslin, Caprica-Six and her child Hera in the Kobol Opera House. She later finds out that both Roslin and Caprica-Six deal with the same dreams, but their meaning remains unclear at first (TRS: "Crossroads, Part II").
Along with her husband, Agathon is among the crew of the Demetrius (TRS: "The Ties That Bind"). 58 days into their mission to find Earth, Sharon Agathon and others show concern that Thrace cannot be trusted, and that they have to meet up with the Fleet for refueling (TRS: "The Road Less Traveled"). She physically restrains Thrace to prevent her from executing an FTL jump to a Cylon fleet. When Thrace decides to take a Raptor instead, Agathon accompanies her assuring her that she can be trusted despite her mutinying. On the basestar she meets other Eights who ask her to lead a mutiny against Natalie, but Agathon is disgusted at their lack of conviction, saying that they have to stick with the side they chose. During the attempt to get information from the Hybrid, another Eight is shot by a Centurion. After the Hybrid gives Thrace a cryptic message, Agathon disconnects the Hybrid herself, but refuses to take the mortally wounded Eight's hand and offer her forgiveness and comfort as she dies. Anders does instead and once the baseship jumps back to the Demetrius, Agathon contacts her husband lets him know that they have control of the ship (TRS: "Faith").
After they return with the Cylon rebels, Agathon has another vision of the Opera House. She wakes up to see her daughter standing next to her bed and finds out that she has filled an entire scrapbook with drawings of a blond woman and the number "six". Shocked at the drawings, she notices that Hera has left the room and runs out into the hallways. Her search for Hera on Galactica parallels the Opera House visions. At the same time Natalie is escorted to a meeting with Admiral Adama and she and her guards run right into Hera. Fascinated, Natalie kneels down to be close to the child. When Agathon catches up to them, she draws her sidearm and tells Natalie to back off. Then, after ensuring that Hera is taken to safety, Agathon shoots Natalie, mortally wounding her, fearing that she would take her daughter from her, like the Six in the vision (TRS: "Guess What's Coming to Dinner?").
After the shooting she is taken to see Admiral Adama who demands an explanation from her. She describes the vision to Adama who finds the explanation from her inadequate and decides to send her to the brig. She requests to keep her child, but Adama declines, believing that the brig is no place for a child. After Adama goes through his own emotional turmoil and making questionable decisions vis a vis the abduction of President Roslin and many of his pilots by the rebel Cylons, he instructs Colonel Tigh to give her Hera back despite her being imprisoned (TRS: "Sine Qua Non"). She is later released from the brig to assist with a rescue plan during a hostage situation aboard the Cylon rebel baseship.
When the Fleet finally arrives at Earth, she is with her family in her quarters. She and her husband are among the first team to land on the planet, but, to their despair, only discover a destroyed city (TRS: "Revelations").
Athena is incarcerated in the Cylon brig with her husband, her daughter, Caprica-Six, Tigh, and Anders during the mutiny, and is released by Starbuck and Apollo to help put down the rebellion (TRS: "The Oath, Blood on the Scales"). After Boomer returns and escapes imprisonment, she assaults Athena and steals her uniform to impersonate her and abduct Hera (TRS: "Someone to Watch Over Me"). Even after a rescue mission for Hera is put into motion, Athena remains emotionally devastated as she does not expect to find her child alive (TRS: "Daybreak, Part I"). Athena, Helo, Starbuck, Apollo, and a force of free Centurions board the Colony to retrieve Hera. Boomer returns the child to her parents after a change of heart. Athena then shoots Boomer dead, which Boomer expected. The Agathon family settle on the newly discovered Earth. Athena is a direct ancestress of all modern-day humans through her daughter Hera (TRS: "Daybreak, Part II").