Zak Adama is the dead son of Commander William Adama and the younger brother of Lee. He was killed in an flying accident shortly after graduating. He was engaged to Kara Thrace, who was also his flight instructor.
While Zak is never in the comic series and makes really tiny appearances in flashbacks, his influence in his father, brother and Starbuck’s life helped define who all three of them are.
In the comic series, a Returner (a cylon who looks and has all of the memories of a dead member of the colonies) of Zak shows up.