Brainiac Pac is the form Pac takes when he consumes what's known as the Brain Berry. He takes on a nerdy look (including buck teeth, glasses, and a spotted bow tie) and a very high IQ. This in turn allows him to invent a machine to combat Dr. Buttocks' giant pseudo Grinder robot.
Cylindria takes quite a liking to Pac's Nerdy form, stating "Is it just me? Or does Brainiac Pac look kinda cute?"