Mr. L
An extra playable character? No, it's just Luigi in disguise! You don't know he's Luigi, though. In fact, even he doesn't. This is his brainwashed villain alter-ego from Super Paper Mario, where he's persuaded to the enemy side by super hypnosis.

We don't have to worry about any of that in RMN World, though. He has his couple of levels and causes no further trouble for the other characters. Besides, he's still just Luigi after all.