Ah-ha-ha-ha-ha-ha!!
Hacker is Doc Terror's right hand man. Often Doc Terror orders Hacker to do his dirty work for him.
During a flashback in "Man or Machine (Part II)" he was once a criminal who was running away from the police, after evading the cops, Hacker met Doc Terror who offered him money and power if he worked on his side, Hacker agreed and becamed the Doc's experiment of turning a human into a robot, right after the experiment, Hacker regreted and during his moments of anger, he accidentally injured Doc Terror, which later became a cyborg because of that.
In the episode "The Better Half (Part I)", Hacker is the (involuntary) responsible of the creation of Uniborg, by accidentally pushing his Legion half, getting it close enough to Terror's Syntax half, so the units could fuse into one robotic being.