Some TMNT stuff really isn't for little kids.
In the Teenage Mutant Ninja Turtles & Other Strangeness role playing game by Palladium Books, Karl is the chief laboratory assistant to Dr. Victor Oban Feral in Feral's secret underground laboratory. Karl is a core BIO-Spawn character, and also has roles in a scenario published in Teenage Mutant Ninja Turtles Adventures!.
Karl is a mutant rabbit of average human size, height, weight, intelligence, speech faculties and manual dexterity. Unlike Otto Rattus, Karl has not been otherwise modified to pass for human, and bears the same fullness of animal appearance as Splinter and his sons.
Karl wears simple clothes: a pair of shorts, a pair of overalls, a chain improvised as a belt, and he goes barefoot. He regularly wears a set of spiked brass knuckles to discipline the other animals in the lab. Karl is also surgically fitted with one of Feral's control collars in case Karl is disobedient.
Karl may be the chief assistant of Feral's secret underground laboratory, but he hates it, and he hates Feral, and the only thing keeping Karl from trying to escape is the control collar Feral has surgically installed in Karl's neck. This is not to say Karl is that sympathetic a character either, for he is also genuinely evil and does nothing to disguise it. He is arrogant, mean-spirited, and takes joy in inflicting suffering. But he's also quick to obey Feral and agree with him, knowing that Feral is capable of killing him in an instant. Karl also strikes a balancing act by trying to appear intelligent but not too intelligent, so as not to gain enough of Feral's curiosity to end up on the vivisection table himself.
As a sentient being, Karl has average intelligence, but has managed to absorb a college education through his years of on-the-job laboratory experience in Feral's servitude. Karl has become a skilled doctor and computer scientist, with a useful knowledge of chemistry, biology and diagnostics.
Karl began his life as an ordinary rabbit, but became a humanoid mutant through Feral's experiments. Since Feral does not respect animal rights at all, Karl has been a slave in Feral's secret underground laboratory.