SFH Bowser
Bowser was a character who joined Mario, and his goon friends for revenge in Sonic For Hire episode The Battle: Part 01, and Part 02. He fights against Earthworm Jim, and he died from Earthworm Jim's ray gun. His death was mention by Sonic when Princess Peach asked for Bowser's whereabouts. It is unclear why he joined forces with Mario, although it is possible that he has some grudge against Sonic.