Zak Adama (alternate)
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Adama was engaged to Kara Thrace, his flight instructor. Due to her feelings for Adama, she passed him, despite her knowing that he was the "worst flier I ever trained". As proof of this, Adama has a scar on his left hand, which was a result of slamming his hand as he closed a Viper cockpit during his first day of flight training. However, his one strength was that he was a better shot than Kara Thrace (Battlestar Galactica 1).
On his graduation day from the Colonial Fleet's training center, his father told Zak that he was a disgrace. This was prompted by Zak's calling his father "dad", instead of the customary "sir" (Battlestar Galactica 2).
Zak the Returner
A clone of him believed to be a "Returner" is found by Kara Thrace aboard Medivac 12 (Battlestar Galactica 0). Despite knowing that the human Zak was dead and buried, Thrace helps him ward off an attack by new-model Centurions on Medivac 12 (Battlestar Galactica 1).
After piloting Medivac 12 away from debris, he escapes to Galactica in a zip pod piloted by a member of the Earth Protectorate, who is paid for his services by ardent believers of the Sacred Scrolls. Zak is later captured, having inadvertently infected the worshippers with a virus, and is summarily interrogated by Lee Adama.
In the course of the interrogation, Lee slips the fact that he has feelings for Kara Thrace, which results in a fracas that allows the virus to infect Lee. However, during the fight, Zak saves Lee from an Earth Protectorate assassin and later accompanies Lee in taking out a group of Protectorate terrorists.
Lee succumbs to the virus' effects, which enrages Commander Adama, who assaults Zak. However, Lee mentions that Zak saved his life and the elder Adama begrudgingly accepts the fact that Zak could have killed either one of them several times over.
When Lieutenant Felix Gaeta informs Zak and Commander Adama of the approach of an antebellum basestar, the elder Adama is forced to rely on Zak's flying skills in order to attack the basestar (Battlestar Galactica 2). It is before this attack when Thrace gives Zak a callsign, Buster, as a result of busting three of the test maneuvers that should have resulted in his obtaining a failing grade. It is after this that Commander Adama informs Zak that he should have never passed basic flight, and that he was passed due to Thrace's feelings for him.
During the attack, Zak tries to inform Commander Adama about the demolition gel on the basestar, but the commander deploys the nuclear warhead. The resultant explosion causes damage to Thrace's Viper.
Realizing that he's somehow a Cylon construct, he doesn't surrender himself to this, but instead exchanges his life to save Thrace by swapping Vipers. Now in Zak's Viper, Thrace is able to leave the area as her damaged Viper explodes, killing Zak (Battlestar Galactica 3).