"He's a mercenary for hire and master of disguise."
Dennis the Rabbit, named "Mr. Cutie Patootie" by Candace Flynn and Mr. Bigelow by Major Monogram, 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 Agent P's lair and, by extension, the O.W.C.A. mainframe, he let Candace see him. When Candace found him beside the street, she named him 'Mr Cutie Patootie' and raised him as a pet. Candace used him as a living doll for dress-up and taught him a couple of tricks, inadvertently showing his obsession with carrots. He engaged Agent P in multiple fights before managing to infiltrate Agent P's lair and trap him in a cage. While Dennis was attempting to hack into the organization, Agent P took advantage of the rabbit's love for carrots and defeated him by dropping a truckload of carrots into the lair. Dennis was likely taken into captivity by the O.W.C.A. ("No More Bunny Business").
Like many rabbits, Dennis has a strong fondness for carrots. He 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.
- Dennis growls as his only vocalization, reminiscent of Agent P's own chatter. He also gasps as well as growls in "The Return of the Rogue Rabbit".
- Despite his inability to recognize Agent P (either if he's wearing his hat or posing in 'mindless pet mode'), Doofenshmirtz is perfectly able to see through Dennis' 'mindless pet mode' and warns Dennis that he can't be fooled by it.
- He and Pinky are the only known agents to have a name that is not an alliteration, such as Perry the Platypus, or Peter the Panda.
- It could be that he is an allusion to Alec Trevelyan from the James Bond movie GoldenEye.
- "No More Bunny Business"
- Hey, Where's Perry?
- Phineas and Ferb: Across the 2nd Dimension video game (Mentioned only)
- "The Return of the Rogue Rabbit"