Enzo is a former drug supplier and criminal that used to work with Kat prior to the Cylon attack.
During the episode "The Passage", Enzo recognized Kat on the Galactica and called her by her real name "Sasha". Starbuck witnessed the altercation and confronted him later. Prior to Kat's final mission, after she had already received a lethal dose of radiation, Enzo and Kat had one final moment together.