|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
Dr. Broccoli is the self-proclaimed greatest rival of Princess Bubblegum, despite the fact that nobody has ever heard of him before nor has Princess Bubblegum ever mentioned him before. He attempted to destroy Princess Bubblegum's Silly Sting Theory Device with his Brussels sprout robots in order ruin her appearance at an science conference. He was stopped when Jake brought the shape-shifting alien Blergal to pretend to be his mother and make him eat his Brussels sprout robots and go to his room.