Zac was born from a chem spill in Zaun's Sump level, gaining basic sentience to respond to outside stimuli. He was sequestered in a laboratory by a group of alchymists funded by chem-baron Saito Takeda, to be molded into a supersoldier under his command. Two of the alchymists grew emotionally attached to Zac and, when they discovered who their benefactor was, spirited away with him deep within Zaun, lovingly raising him as their own son while being on the run from baron Takeda's thugs.
Zac responds to people's emotions and acts accordingly, his mood changing with the reigning sentiment at any given time, best exemplified when he went berserk after his parents were killed by Takeda's men. After burying them, he decided to set off and help the people of Zaun, fighting chem-thugs and fixing the city's infrastructure when the opportunity presents itself.