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Nixon (or Nix as Harley liked to call him) was a fairly loyal, if not begrudging, member of Harley's Quinntets. However, he eventually became fed up with Harley's schemes and attempted to kill her and take control of her gang, which he felt could run Gotham without her as their leader. Harley ended up killing him by jamming explosive chewing gum into his gun.
When Harley ended up dying and being sent to Hell, she was re-united with Nix and the rest of the Quinntets, who had been stuck in an infinite loop of attempting to survive a SWAT team assault led by the deceased and demonically-appointed Detective Pettit. Despite the hopelessness of the situation, Harley refused to stay in Hell, and so she rallied her Quinntets and sought to have them all escape Hell. After several failed attempts, Harley and her team manage to reach the River Styx and nearly escaped Hell. Unfortunately, they were being pursued by the Hell-enforcer, Ulysses Highwater, who ended up killing them and forcing them to start over once again.
Although Harley managed to escape Hell eventually, it's presumed that Nixon is still there.