Dennis the Bunny, named "Mr. Cutie Patootie" by Candace Flynn, is a former agent of the O.W.C.A. who went rogue and attempted to hack into the mainframe of the organization. According to Major Monogram, he is a master of disguise, proven by several images of Dennis that are sinister, followed by his harmless "mindless animal" look.
In order to gain access to the Agent P's Lair and, by extension, the O.W.C.A. mainframe, he lets Candace see him. She immediately attempted to adopt him. Candace used him as a living doll for dress-up and taught him a couple of tricks, inadvertently showing his obsession with carrots. He was defeated by Agent P, whose lair he had infiltrated to hack into the organization, and likely taken into captivity by the O.W.C.A. ("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.
Dennis is a white bunny. In his "mindless-pet mode", his eyes are enlarged and he assumes a quadrupedal position. When not needing the disguise, his eyes are focused and he stands upright like other agents. He is slightly taller than Perry in this mode. Some of his other disguises include a biker outfit, a ninja suit, and a pirate costume. Candace had him wear a purple tutu when she adopted him.
As a former agent of the O.W.C.A., Dennis is a highly trained combatant, with knowledge of disguise, and knowing several different styles of fighting including a variety of martial arts. He has a "mindless-pet mode" like Perry and Pinky the Chihuahua and has shown some skill in hacking. His skill was sufficient enough to injure two O.W.C.A agents, and even trap the highly skilled Agent P. Had Agent P not exploited the bunny's obsession with carrots, Dennis would have succeeded in his mission.
He can dance, being able to do the Worm and the Robot. Dennis can also quickly consume large amounts of carrots.
Major Monogram presents Agent P with a more professional drawing of the rogue agent, Dennis.
Dennis wearing a pirate costume.
Dennis dressed as a ninja.
Dennis in "Pet Mode".
Perry spies on Dennis and Candace.
Candace searches for clothes to put on "Mr. Cutie Patootie."
Dennis looks at the blueprints for a secret entrance.