Kenko the Shapeshifter

From Ben10versepedia

Kenko appeared as the villain of the Sumo Slammer Smackdown video game in "Game Over". He is a shapeshifter and archenemy of Ishiyama, the hero of the game. He overhears Ben talking about the Upgrade token; believing it could be used to allow him to take over the real world, he attempts to obtain it. While Ben searches for the token, he fights and defeats Ishiyama, then assumes his form after Ben reaches the Upgrade token first. He tries to trick Ben into taking him to the real world, but Max's timely intervention in the game reveals his ruse. As Upgrade, Ben's control over the game allows him to depower Kenko, revealing him to actually be a small, weak, gnomish creature. Afterwards, Kenko is defeated when he knocks himself out by accidentally hitting himself in the head with a piece of his own armor.