Head on over to the UnMario Wikia. Be sure to register an account if you haven't already.
Pikachu
From Unmariowiki
Pikachu is an advanced blob of cheese modeled to look like that of a pokemon. He first appeared in Bob The Builder's speacial game, Can We Fix It? In this game, pikachu acted as the 4th boss of the game, with mario appearing next in line. Mario beat up pikachu one day when pikachu was picking berries, so they are acctually mortal foes. Although Pikachu is a piece of Cheese, no charecter has ever tried to bite this majestic creature.
As Luigi observed in an unaired episode of SMK Radio "Pikachu" rhymes with "peek at you".