Zack is a close ally of Cloud's, a member of SOLDIER from Gongaga. Formerly under his superior officer Angeal, who gave him the Buster Sword.

It was Zack who gave Cloud's life hope in continuing his dream of becoming First Class SOLDIER. He was also Aerith's boyfriend. Zack was also known to be Sephiroth's best companion before the incident in Nibelheim. After that, both he and Cloud were subjected to Hojo's "Sephiroth Copy" product for the next few years in Shinra manor.

Zack managed to get himself and Cloud out and attempted to enter Midgar, where Zack intended to be a mercenary for hire. However, he sacrificed his life to save Cloud from Shinra MP gunfire. After that, Cloud, due to a mix of his personal issues and Jenova's influence, adapted many of Zack's mannerisms and as such used the Buster Sword.

Zack's spirit aided Aerith in the Meteor incident and later in saving Cloud after his near-death experience in Advent Children. By Advent Children, Cloud used the Buster Sword as Zack's gravestone marker on the very piece of land Zack died on.