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Julian "Marksman" DiMarco is formerly the commanding officer of Columbia (Battlestar Galactica: Season Zero 0), in addition to being a former Viper pilot (Battlestar Galactica: Season Zero 1). He later attempts to overthrow the Colonial government, believing it to be corrupt and in need of restoration (Battlestar Galactica: Season Zero 4—6).
William Adama serves under DiMarco on Columbia as that ship's executive officer for a number of years, where they became friends. One of his noted sayings is "Stay young or die, and never compromise".
A ship he was on, having left Colonial space and into Neutral Space, was deemed lost and Adama was given the mission to search for it and DiMarco (Battlestar Galactica: Season Zero 0). Adama later discovers that DiMarco is responsible for bombing a colony of Colonial separatists, who himself found out that the Expeditionary Fleet was actually a concoction of the military as an umbrella for black operations. Out of grief for what he had done, DiMarco deliberately crashed his ship so that people would assume that he and his crew are dead, willing to live the rest of their lives on the same planet he had bombed (Battlestar Galactica: Season Zero 1). However, according to DiMarco, the Fleet later sends a "kill squad" to the planet that slaughters a majority of his crew in an attempt to drive him from the planet (Battlestar Galactica: Season Zero 6).
He later marries Sil, one of his crew on the survey ship, and with her begins to organize a terrorist organization to strike the Colonial government. However, this falls apart after Christa Nolan kills Sil—who was secretly wanting to betray him as she viewed him as insane—and DiMarco assaults Nolan, which dissolves the deal between himself and Darja Purat, his weapons supplier (Battlestar Galactica: Season Zero 5). After Kara Thrace and Karl Agathon manage to cause serious problems for his rebellion, Purat and Nolan desire to leave: DiMarco kills them.
DiMarco and a handful of his surviving resistance fighters manage to extricate themselves from the base in a ship. Adama manages to board the ship before take off and discovers tha DiMarco has a Viper Mark II fitted with a nuclear warhead. DiMarco's intent to nuke the Colonial Military Complex on Sagittaron, with the hope of showing the Fleet their handiwork "up close", is stopped by Adama, who intercepts him in another Viper from the rebel ship. Unwilling to kill Adama in a firefight, DiMarco congratulates Adama on how good a soldier he is, and ejects himself into space without his helmet, thereby committing suicide (Battlestar Galactica: Season Zero 6).