Dennis, named "Mr. Cutie Patootie" by Candace, is a former rabbit agent of The O.W.C.A. who went rogue and attempted to hack into the mainframe of the organization. Major Monogram tells Agent P that he is a master of disguise, showing several images of Dennis that are sinister, followed by his harmless "mindless animal" look. In order to gain access to the Flynn-Fletcher household he lets Candace see him. She immediately attempted to adopt him. Candace used him as a living doll for dress-up and taught him tricks, inadvertantly showing his obsession with carrots. Linda spots Perry and Dennis battling, but on her double-take, sees both posing as mindless animals. She seems unconcerned that Candace has adopted a pet without her consent.
He was defeated by Agent P, whose lair he had infiltrated to do so, and likely taken into captivity. ("No More Bunny Business") Like many rabbits, Dennis has a strong fondness for carrots, and has even interrupted his mission in order to eat carrots, which eventually led to his capture. He eats so many carrots he cannot fight back when Agent P hand-cuffs him.
Dennis growls as his only vocalization, reminicent of Agent P's own chatter.
Carl dresses up as him whilst he tries on disguises in ("Undercover Carl").