April O'Neil: Rockwell developed a soft spot for April when he attacked her and Donnie while they were investigating his disappearance. It was revealed through her half-Kraang powers that that the mutant chimpanzee she and the Turtles were up against is Rockwell and that his colleague Falco has more to tell about the experiment with Mutagen.
Donatello: At first, in Monkey Brains, after Rockwell was mutated into a chimpanzee, he barely attacked Donnie. In Battle for New York, Rockwell became part of The Mighty Mutanimals and Donnie saw that Rockwell can talk.
The Kraang: Rockwell was assigned by the Kraang to work with Falco on an experiment involving Mutagen, but he wanted no part of it. As a result, they helped Falco mutate Rockwell and use him his experiment, but Rockwell intends to get them to pay by braking their rule over the city with the Mutanimals.
Dr. Victor Falco / Rat King: Rockwell and Falco were colleagues and lab partners until Falco mutated him into a chimp by testing his neurochemical on him.
Tiger Claw: Rockwell and Tiger Claw don't interact much, but he was captured along with Slash by the mutated bounty hunter to be experimented on.