Mike Believe (a pun on the phrase make believe) was the new student at Pokey Oaks in "Imaginary Fiend", an episode of The Powerpuff Girls.
Mike seemed to be new in town, as the class didn't seem to know him, and Ms. Keane introduced him to the class at the beginning of the episode. He really wants a friend, so he creates Patches, an evil ratty-wizard-type person, who causes trouble for the whole class.
Blossom finally figures out (after the girls try and fail at fighting him) that they have to create an imaginary friend to get Patches out of there. The girls create a bunny, who wears a pretty dress and combat boots and who has powers similar to the girls, along with a sweet, motherly voice. The Bunny fights Patches and sends him on his way. Bubbles tells Mike (after he explains that all he wanted was a friend) that the girls will be his friends.
Mike is voiced by Aaron Spann, who was 10 years old when the episode aired, making him the youngest guest star or actor in general to do a voice-over on The Powerpuff Girls.
He appears standing behind Kim, Joey, Jeff, and Mary at the beginning of "Mojo Jonesin'".