Zak Adama was the younger son of Commander William Adama, and, with both his father and his big brother Lee to look up to, took numerous risks. One, sadly, went catastrophically wrong, resulting in his death. The acident was deemed "pilot error", but Cmdr. Adama, Lee and Kara all blame themselves and one another at certain times and to differing degrees.
Zak's death drove a wedge between his father and brother, and as the mini-series begins they haven't spoken since his funeral, some two years prior. Zak was romantically involved with Kara, who was his training officer, and who confides in Lee that she should have failed him (Zak), since he was not up to scratch as a pilot.