D.A. Sinclair was a genius, but arrogant and misguided, college student. He used his intellect to create cyborg-zombie killing machines.
When he converts Invincible‘s classmate, Rick Sheridan, into a Reanimen, Invincible and William track him down and confront him. Invincible defeats his Reanimen, and manages to partially restore Rick’s mind, before knocking Sinclair out and turning him in to Cecil.
Despite Invincible’s expectation that Sinclair would be incarcerated, he was instead offered a job by Cecil to use his intelligence for the government. After the Invincible War he was instructed to turn the corpses of the dead Invincibles from other dimensions into Reanimen.