From the Super Mario Wiki
Mr. E is a Toad from Mario Party Advance. Mr. E's name is a pun on the word "mystery". He is a regular Toad who has large eyebrows and wears a purple jumpsuit instead of vest and diaper-like pants. He loves unsolved mysteries, and often appears during exciting events of Mario Party Advance, though he proves himself to be quite cowardly, constantly running away before the action starts in several of the quests involving him. Despite this though, he continues to seek mysteries throughout the course of the game, assisting characters baffled by various mysteries as if it is his job to do so. He is also very prideful and considers himself superior to the player, referring to the current character as his apprentice even though he himself does little work in solving the mysteries he gets the player involved in. According to the game's ending credits, the Mr. E of Mario Party Advance mysteriously disappeared after that game's events, no one knowing his current whereabouts.