From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Messengers are the curious spectral apparitions seen by Gaius Baltar and Caprica Six, as well as a few other characters. These beings were earlier referred to as "head characters", i.e. "Head Six" and "Head Baltar", by internal documentation and scripts. These messengers often present themselves as someone known very closely to the subject in question, and appear in the employ of a higher power who does not care to be referred to as "God."
"Torn" establishes the humanoid Cylon visualization process known as projection. In the Miniseries and Season 1, before the term "projection" was introduced, the character of Gaius Baltar has frequent visions of an image of his Cylon love interest from Caprica, now known as Caprica Six.
Neither character directly reveals the presence of these images to each other or any other character. The images appear to Six and Baltar together during the Battle of the Colony, revealing the truth to the pair. Occasionally both Baltar and Caprica Six have been viewed reacting to their messengers, either by talking to themselves or by physical reaction, though this has generally been discounted as general "oddness". Baltar makes reference to seeing angels while speaking on the wireless as a cult leader, and when convincing John Cavil to stand down during the Battle of the Colony (TRS: "Islanded in a Stream of Stars"; "Daybreak, Part II").
The personality of each image is often represented in the clothing that they wear. Messenger Six often appears in a revealing red dress, and the Messenger Baltar appears wearing Gaius Baltar's custom-tailored, pinstripe suit (TRS: Miniseries, "Downloaded").
Each messenger shares a few personality traits of the person they purport to represent but with some key differences. Being of possible divine origin, both are calmer and wiser than their corporeal counterparts. While Messenger Six is as mischievous and sexual as Caprica Six, she espouses Cylon philosophy and religion in a zealous, unwavering manner. Likewise, Messenger Baltar shares Baltar's suave arrogance but often criticizes Caprica Six when she attempts to rationalize the destruction of the Twelve Colonies or other Cylon motivations. Both personalities tend to sway its actual opposite to side with their philosophy: For Messenger Baltar, the need to become more human (and, like the actual Baltar himself, selfish); for Messenger Six, worship of the Cylon God and distancing Baltar from any opportunity to give support to the Colonial causes.
In the last days of the journey, these messengers are revealed to be agents of a higher power— "angels of God," something previously claimed by Messenger Six to Gaius Baltar in both "Home, Part II" and "Torn"; writer Jane Espenson has stated that she personally believes they are "a far-advanced culture with an interest in humanity." This likens them to the Beings of Light from the Original Series mythology. (TRS: "Islanded in a Stream of Stars"; "Daybreak, Part II").
Baltar initially believes that Messenger Six is a hallucination in its first few weeks of appearances (TRS: Miniseries, Night 1, "33"), but later believes the Messenger's initial suggestion that she is generated from a Cylon cybernetic implant. He disproves this notion when Doctor Sherman Cottle performs a MRI scan, showing no abnormalities (TRS: "Home, Part II").
Messenger Six demonstrates awareness of events, people and places that Baltar does not appear to know consciously. She tells him of a Cylon transponder hidden in Galactica's CIC (TRS: Miniseries, Night 2), and appears to aid him in his conversion from atheism to Cylon monotheism through guiding him in how to destroy a Cylon tylium refinery (TRS: The Hand of God). The source for Messenger Six's data is initially unclear, but is later assumed to be through an act of divinity.
Baltar's visions of Six not only have the Cylon appearing with him on Galactica, but the two also envision themselves in Baltar's now-destroyed home, on the beach, and other locales. Messenger Six seems to be able to force Baltar to see additional items in his environment, such as the human skulls on Kobol, as well as visions of the interior of the ancient Opera House and an image of what Messenger Six believes to be a Cylon-human hybrid child. When an actual child does appear in the Fleet, the Messenger Six claims it is her vision fulfilled. Baltar is convinced by this that she cannot be a hallucination, but Cottle's scan appears to have ruled out the possibility that she is an implant. He asks her who or what she is, and she replies that she is an angel (TRS: "Kobol's Last Gleaming, Part II", "Valley of Darkness", "Home, Part II").
Over time, Baltar is seen by some characters speaking to himself, which is often the case when he is speaking to Messenger Six. In several instances, Messenger Six appears to Baltar to be a physical entity, grabbing him by his clothes or body, or assisting him with objects ([[TRS: "Six Degrees of Separation", "Taking A Break From All Your Worries"). She has even demonstrated an ability to physically interact with Baltar. On one occasion, she pushed him into a bulkhead and grabbed him by the throat. In another example, she lifted him into a standing position after he was beaten by a Galactica marine (Escape Velocity). In both cases, this provoked reactions from other people. Further, she appears to satisfy Baltar's sexual urges, leading to awkward situations where he is seen masturbating (TRS: "Colonial Day").
The Messenger Six has been present when Caprica Six is near only twice. She is also conspicuously absent during Shelly Godfrey's tenure on Galactica (TRS: "Six Degrees of Separation"). The Messenger Six seems to, for the most part, vanish during the period when Caprica Six and Baltar renew their relationship on New Caprica. When Baltar is aboard a Cylon basestar, Caprica Six explains the projection process to him while the Messenger Six hints that this process is very similar to how she and Baltar communicate and that Baltar may himself be a Cylon (TRS: "Torn"). Messenger Six also appears to Baltar, when he discovers Hera under Maya's dead body and disappears when Caprica Six comes near Baltar and the baby (TRS: "Exodus, Part II").
Messenger Six has a musical theme, a simple 9-note motif with a staccato xylophone and some violins, which plays behind almost all her appearances. This theme also plays without Six present in "Rapture" after D'Anna Biers collapses in Baltar's arms after her vision of the Final Five, until Baltar is pistol-whipped by Galen Tyrol.
From the end of Season 2 on, Messenger Six claims more often to be an angel from God.
Ultimately, Messenger Six is revealed to be a very real being -- not a projection of something inside Baltar's mind -- sent by another being that may be God. Both her and Messenger Baltar appear to both Caprica Six and Baltar during the Battle of The Colony; Baltar and Caprica Six are each shocked that the other can see them. On Earth, Messenger Six, along with Messenger Baltar, reveals to Caprica and Baltar that their destiny had always been to protect Hera, who was the future of both races. They leave, saying that while God's plan is never finished, the two mortals' lives will be much less "exciting" from now on.
One-hundred fifty thousand years later, Messenger Six is with Messenger Baltar in New York City, looking over the shoulder of a man reading a National Geographic magazine article about the remains of Mitochondrial Eve -- the current population's latest common evolutionary ancestor, Hera Agathon. The two discuss how all of this has happened before, and whether all of this will happen again. Messenger Six believes things will go differently this time. Messenger Baltar chides her when she refers to God, saying, "You know it doesn't like that name." The two then walk off into the teeming crowds of New York City (TRS: "Daybreak, Part II").
When a messenger with Gaius Baltar's visage appears to Caprica Six, the notion that the actual Gaius Baltar was part of a Cylon plan to manipulate him via Messenger Six is revealed to be highly unlikely, as other Cylons are unaware of him and do not mention the plan or its effects to Caprica Six (TRS: "Downloaded").
The Messenger Baltar's appearance and demeanor is just as much an extreme opposite of the actual Baltar as the Messenger Six's behavior is to Caprica Six.
At her resurrection, Messenger Baltar immediately tells Caprica Six that only she can see and hear him, and not to reveal what she sees to the other Cylons present; she never asks about his origins.
Messenger Baltar confronts Caprica Six with her guilt in the slaughter of billions of human beings, and with the logical error behind the Cylons's actions. Under Messenger Baltar's influence, Caprica Six speaks out against the war alongside another resurrected "hero of the Cylon", Sharon "Boomer" Valerii. The two forge a new quasi-benevolent path, albeit temporarily, for the Cylon race.
Messenger Baltar appears irregularly in seasons 2 and 3. After Caprica Six surrenders herself to the Colonials (having assisted Sharon "Athena" Agathon in her rescue of Hera), Laura Roslin and Tory Foster observe Caprica Six speaking to and kissing Messenger Baltar—who is invisible to Roslin and Foster (TRS: "The Woman King").
While Messenger Six claims to be an angel sent from God, Messenger Baltar is contemptuous of the Cylons' concept of God (TRS: "Downloaded"). He is also described by Ron Moore as more of a "devil" figure in Six's consciousness.
While Messenger Baltar acts mostly as a conversational foil for Six, in "Crossroads, Part I" he informs her that Saul Tigh has suffered the loss of a woman close to him, to assist Six in dealing with Tigh's interrogation (TRS: "Crossroads, Part I").
After his ascension to cult leader, Gaius Baltar encounters his Messenger doppelgänger after running into Tory Foster at the civilian's mess hall. Baltar is initially shocked by Messenger Baltar's sudden appearance, voicing his belief that it is Messenger Six appearing to him as a doppelgänger version of himself, but the Messenger points out that she would have no need to disguise herself from him; Baltar becomes amused by the idea of having himself as a conversational foil for once instead of Six. This Messenger Baltar encourages him to engage in a relationship with Foster, claiming that she is "special," and guides him, after a fashion, through her covert interrogation attempt. This Messenger Baltar does not elaborate on that point, but Gaius Baltar follows the Messenger's lead (TRS: "Six of One").
Prior to the conclusion of the Fleet's journey to Earth, Messenger Baltar and Messenger Six appear together, and both Caprica Six and Baltar can see them, and are each shocked that the other can. He and Messenger Six are representatives of the being that some call God, who Baltar describes to Cavil as a force of nature that is neither good nor evil. Messenger Baltar and Messenger Six make one last visit to Caprica Six and Baltar to tell them that their role had always been to protect Hera, and now that this has been fulfilled, the two Messengers leave them alone. Messenger Six explains that God's plan is never over, but Messenger Baltar tells the couple that their lives will be much less exciting.
The two Messengers visit Earth 150,000 years later in New York City discussing humanity finding its most recent common ancestor, the human-Cylon hybrid Hera Agathon. The two discuss the cycle of life, and Messenger Baltar chides Messenger Six for calling the being they work for "God," as it apparently does not like being called that. Messenger Baltar has the series' last line, which is a response to Messenger Six's stern look: "silly me... silly, silly me." The two then walk off together into present-day New York City (TRS: "Daybreak, Part II").
Laura Roslin converses with a being in the form of Priestess Elosha during what should be zero-time jump sequences on the way to the Resurrection Hub. This Messenger being is unlike the others, in that not only do they interact outside the realm of others, it takes place during a time interval that should not exist. The being seems to have some knowledge of the future events, such as Roslin's death and the fact that William Adama waits for her, suggesting both reality for the being and the out-of-time conversations. Unlike Kara Thrace's Messenger Leoben, the experience is difficult to explain with unconscious dreaming or hallucination (TRS: "The Hub").
The Final Five's Messengers
After recovering his memories of Earth due to trauma resulting from his being shot in the head, Samuel Anders reports that the Final Five were originally warned of impending nuclear destruction of Earth two thousand years earlier by images of people no one else could see. This information was the impetus for their recreation of Cylon Resurrection and the preparing of the ship on which they escaped following the disaster. The one that appeared to Anders looked like a woman, the one that appeared to Tory Foster looked like a man. Much like Gaius Baltar, Galen Tyrol thought he might have a neural chip (TRS: "No Exit").
Kara Thrace encounters a vision of her father going by the name of "Slick". She initially fails to recognize him as her father. As she spends time with him, she recalls playing the piano with her father. She assists him in composing a song and compares him to her father. While she plays the piano with him, she recalls playing the piano with her father as a child. She recalls a certain song, and notices that the score resembles one of Hera Agathon's drawing. As she plays the song with him, Saul Tigh and Tory Foster recognize the song as the same song they heard at the Ionian Nebula. When Tigh grabs her and asks her about the song, Slick vanishes (TRS: "Someone to Watch Over Me").
A Messenger appeared to Zoe Graystone decades before the Fall of the Twelve Colonies in the form of Zoe herself. The Messenger's first act was to save Zoe from a fire which burned down the Graystones' prior residence, when Zoe was a little girl. She then continued to appear to Zoe, who considered her a friend, periodically. Messenger Zoe had the form of teenage Zoe before Zoe herself entered adolescence. On one occasion, when Zoe was a teenager and now identical to her Messenger counterpart, Messenger Zoe encouraged her to outdo her father Daniel Graystone by creating life with her computer skills, after they noticed that Daniel had "stolen" the design for the U-87 Cyber Combat Unit from drawings made by Zoe. This suggestion led to the creation of Zoe-A, Zoe's holographic avatar duplicate (CAP: "Things We Lock Away").
Messenger Zoe later appears to Zoe-A when the latter is engaged in combat with Tamara-A and several users in New Cap City who blame her for the destruction of Maglev 23. She encourages Zoe-A to be her own person and not accept the blame for Zoe's sins. This leads to Zoe-A convincing Tamara-A to form an alliance (CAP: "Things We Lock Away").
An unidentified Zoe sits smiling among Sister Clarice Willow's otherwise entirely Cylon congregation of monotheists in the flash-forward montage at the end of "Apotheosis". The scene precedes Zoe-A's resurrection into the first skinjob body, yet is years after original Zoe's death and Zoe-A's denouncement of Willow.
Image of Leoben that appears to Kara Thrace but never claims to be a Cylon in the episode "Maelstrom." Because he only appears to her in a vision while the pilot is unconscious, and does not fit the Baltar-Six analogue, Battlestar Wiki considers him separately.
For further information, please see Messenger Leoben entry.