|Voiced by||Chris O'Neill|
He is shy but quite known for being best friends with Satan. Although he seems terrified by him, and ends up getting into trouble every time he's with him. For example, In Sugar Trip, both Leo and Satan had a massive hallucination after eating their 'sugar' (which was actually cocaine) covered pancakes. Not only that, but later that day (or some other day), Leo woke up nude on the beach, along with Satan destroying a cruise liner in the background. Another example is when Ron possesses Leo after using a Ouija board and Satan accidentally shredded Leo's upper torso into pieces, which then killed him until he came back through the trap door from hell. (The mechanics of which remain aloof, probably to not to detract from the humor).
A recurring gag is that Leo's age is never given. In Leo Goes to School, his birthday cake (which is a pancake) has hundreds of candles, which adds to the confusion about his age. It can also be assumed that he is just a child, as evident by Satan calling him "Little boy" in various episodes.
In Battery Catastrophe, Leo seems to be somewhat of a gamer, playing video games with Satan. It was revealed in Leo Goes to School that Leo has never been to school before, despite the fact that he had algebra homework in Algebra Aversion. Its was shown that Leo's parents are dead, at least until Leo wished for his parents to be alive.