From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Believing himself to be a great leader, Xaviar differs with Commander Adama on how to best bring Earth's technology up to the level of the Colonials. He believes that by using the time warp synthesizer technology developed by Doctor Zee, he can lead a team back into time to advance Earth's technological development from the past, so that Earth is capable of defending the Galactican Fleet in 1980 A.D. After failing to solicit Adama's support for his plan, who obliquely rebukes Xaviar from even presenting his plan to the Council for a vote, he takes matters into his own hands and goes back in time to try to alter history and accelerate Earth's technological development by himself.
Xaviar goes back to Nazi Germany to try to introduce advanced weapons technology to the Germans via their V-2 rocket program, which is thwarted by Troy, Dillon and Jamie Hamilton, who pursue him into 1944. After successfully evading the trio, he returns to 1980 and attempts to co-opt Professor Donald Mortinson, a nuclear physicist contacted by Troy and Dillon, to participate in his time travel plan.
The storyline implies that Xaviar also goes back to Revolution-era America to attempt to change history there, a scheme that also fails thanks to Dillon, Troy and Hamilton (1980: "Galactica Discovers Earth").
Xaviar later returns to the present time frame in a plot to try to ransom Galactica's children in an attempt to secure his freedom. Using a process known as epidermal transformation, he takes the form of Lieutenant Nash and convinces Troy and Dillon to take a booby-trapped Viper in pursuit of Xaviar as allegedly ordered by Adama. After they are out of the way, he tries to take the Super Scouts and Hamilton hostage after a baseball game, but is thwarted by Troy and Dillon. Although he escapes, he shoots Colonel Jack Sydell (1980: "Spaceball") which lands him in the hospital in a coma-like condition (1980: "The Night the Cylons Landed, Part I").
In the unproduced scripts
In "The Day They Kidnapped Cleopatra", a script that was being shot at the time Galactica 1980's cancellation order is handed down by ABC, Xaviar travels to 48 BC and attempts to change Earth's history from Egypt by convincing Cleopatra that he is a god, via the use of his invisibility field. After Cleopatra has his drink poisoned with asp venom so as to prove that he's indeed a god, Xaviar makes a fearful flight to 1980 AD to solicit the help of Troy, Dillon, and Jamie Hamilton, bringing Cleopatra along with him.
After the poison is neutralized by 1980 Earth's medicines, he escapes the Warriors and, with Cleopatra, returns to 48 BC. Troy, Dillon and Hamilton pursue him and thwart his plan to take over Cleopatra's kingdom, which fell some 18 years after, as stated in Earth's history.
In an episode that was written as a sequel to "The Return of Starbuck", Xaviar returns to 1980 and is pursued by a squadron of Vipers. During the pursuit, a phalanx of Cylon Raiders interfere and attempt to capture Xaviar, desiring to know why the Colonials are pursuing one of their own. Xaviar escapes back through time, but drags a Raider back to 592 BC. With Troy (who is sent back by Starbuck, now a Being of Light), they destroy the Raider before it can do any damage to Earth's past, and Xaviar evades capture.
In the novelization by Michael Resnick, Xaviar is described as having gray eyes, and is both a warrior and a scientist.. Despite his initial claim to want to deliver an Earth that can defend itself against the Cylons, Xaviar seeks to live a luxurious life (with harems) while overseeing an empire.. His first attempt to alter Earth's technology occurs in June of 1944, having arrived one month earlier and is able, in that short time, to gain a spot on one of Germany's most important projects. However, Xaviar is thwarted by Troy, Dillon, and Jamie Hamilton, and they pursue him throughout the various eras (since Xaviar stole 6 historical texts from Alfred Mortinson's private library), until Troy surmises that Xaviar has given up influencing Earth's technology in the past because the Galacticans have the ability to stop any attempt with their technology, and can fix any of Xaviar's changes in Earth's timeline before they could even occur.