Professor Nibbles is the villain in the show "Katt Karr". He wears a mouse costume and only makes an appearance in "Ferb TV".
According to the episode, his nemesis (played by Roger Doofenshmirtz) and a sentient car resembling a cat called Katt Karr are hunting down Professor Nibbles for some unknown crime he committed. As both Roger and Katt Karr catch to up him driving in his motorcycle, Nibbles takes out a ball of yarn and throws it out, knowing that cats can't resist playing a ball of yarn. This distracted Katt Karr to chase after the ball of yarn, much to Roger's anger. However, in the end, Roger and Katt Karr inexplicably manage to stop Nibbles in his tracks and he is sent away to prison.