After being captured by the Union, Julian Chase was subjected to numerous experiments and brainwashing practices, eventually turning him into the monster known as Nemesis. At some point, the Union offered Nemesis freedom from his Holon and a return to his body in exchange for his services, a deal that he accepted. He would remain affiliated with the Union for the rest of his life.
Nemesis was immensely resentful towards his copy for "stealing" his body and life, and the Polity, for replacing him with the copy. He frequently expressed the almost instinctual desire to kill his copy, even claiming to have no choice in the matter. He exploited their connection in order to psychologically torment Chase by calling his humanity into question, taking extreme amounts of pleasure in doing so.
When overhearing his plan to buy the rest of the gen:LOCK team time by sacrificing his body, Nemesis purposefully let the plan go ahead long enough to doom Chase to the same fate as himself: a lifetime trapped inside a machine. Afterwards, when granted the opportunity to finally kill his copy, he instead left to wipe out the rest of the recruits, showing he cared less about actually killing his copy and more about inflicting as much pain and suffering as possible as payback for the life that he lost.
During the attack on Dallas, Nemesis encountered Cameron MacCloud and ripped her Holon's head off. He proceeded to begin tearing open her Holon in attempt to reach her Mindframe, the experience leaving her traumatized. Later on, when they met again in Atlanta, Nemesis would complain that she "thought too loud" and had not stopped since her birthday, revealing that he could hear her thoughts through their shared connection to gen:LOCK.
Aside from Chase, Dr. Weller is the one Nemesis hates the most. During the attack on the Anvil, Nemesis coldly states that Weller has lost his chance to help him, and refuses any and all offers his former friend makes.
Despite the manipulation and alteration of his mind, Nemesis still held on to his memories with Miranda, and is implied to have still retained some affection for her. During the battle at Dallas, he visibly reacted to her presence and immediately fled the scene. When they met again in Atlanta, Nemesis blocked her escape route and called her by name, before being distracted by the gen:LOCK team. Finally, during the attack on Chicago, when Nemesis was about to finish off Leon August, Miranda stood in his way in an attempt to talk him down, causing Nemesis to stop dead in his tracks and begin glitching out, long enough Chase to arrive and grab his attention.