First, you need to have all of the other characters in the game. Now, start on Arcade Mode, and when you get to the third match win with only an tiny bit of energy left. If you did it right you should hear the word "Great!". The next match will be against Roger or Alex. If you beat them they are yours. Roger and Alex are two joke characters, so only pick them for a laugh. Roger is a kangaroo and Alex is a raptor! To make this cheat a little easier, set the number of rounds to 1 and the game difficulty to Easy.