Dr. Samuel Register shared a lab with Gar Logan
's parents. He was present on the day Logan was infected with Sakutia and turned into Beast Boy.
In the unique cure that prevented Gar from being killed, Dr. Register saw the potential for a universal cure. He worked frenziedly on replicating the experiment, earning the name of the 'Zoo Keeper'. However, Dr. Register was careless with his Sakutia virus. He was infected: The only adult to ever be so. Wanting a cure for himself, and still seeing potential for his universal antidote, Dr. Register came to find Gar Logan.
In the "Beast Boys and Girls" storyline, Register stole a sample of Sakutia to try and mutate it like Garfield's parents did. He accidentally infected himself and every kid in San Francisco. When Beast Boy was infected, the virus in him was canceled out and he was cured.
Zookeeper injected venom from the Sydney funnel web spider into Garfield and attempted to dissect and extract the cure to Sakutia from him. Cyborg and the other Titans kept Register busy while Raven healed Beast Boy. Wanting to stop Zookeeper desperately, Garfield reinfected himself with Sakutia. His powers back to speed, Beast Boy fought and defeated Zookeeper. He also found a way to cure the kids that were infected. Apparently, Register is immune to Sakutia, like Beast Boy.
During the Villains United Infinite Crisis Special, Mammoth broke Zookeeper out of prison. He then joined Alexander Luthor Jr.'s Secret Society Of Super Villains during the crisis.
Some time later, Zookeeper was recruited by Superboy-Prime as part of a new version of the Legion Of Doom, along with Sun Girl, Headcase, Inertia and Indigo.
Powers and Abilities
Dr. Register is able to transform into any animal he can imagine, in a manner similar to Beast Boy. However, his form remains constantly purple.