Aaron was always a disturbed kid; as a child he was often torturing small animals and bullying other children. When Hans Leroy, the leader of the Apocrita assassin group, was passing through and heard of a boy who expertly killed his neighbor's cat, he took Aaron in as his own and attempted to steer his violent nature into something useful.
Of course, Aaron could not be controlled, and grew up without the love of any parental figure. Without any love or attention, his bloodthirst only grew.
Aaron was responsible for leading the secret coup of Apocrita, tricking Simon (Snake) into shooting Hans Leroy accidentally. He convinced the rest of Apocrita that Simon acted out of bloodthirst, and attempted to execute Simon. After Simon escaped, we do not know about what happened with Aaron between then and his reappearance in canon.
Aaron reappeared in America already turned into a superhuman. He gained the ability to stretch his limbs in many different ways, and thus could heal any wound instantly. His weak point was his head; the only part of him that could not mold or stretch. He picked up Darcy as his sidekick and trainee, and proceeded to work as Alpha's right-hand and chief "interrogator".
Aaron had a fascination with pain and torture. His choice tactic of torture involved gallons of acid that he would often douse over one of his victim's shoulders again and again until the bone was revealed, giving his victims a slow death. He also loved torturing with knives, saws, and any sorts of sharp objects, and was despicably cruel to anyone in his clutches. During a session where he was torturing John, he received some of his own medicine: John took one of Aaron's eyes with a knife in an attempt to escape. Shortly after, when John was rescued, Aaron's other eye was plucked out by Creed in a fight, and he was burned alive by Creed and Gabi.
Aaron's death is confirmed.