Mr. L, also known as The Green Thunder or The Mysterious Mr. L, is a brainwashed version of Luigi who appears in Super Paper Mario. He is one of Count Bleck's minions and serves as one of the game's antagonists.
Before turning evil, Luigi tries to stop Count Bleck from using the Chaos Heart; however, after being trapped in Castle Bleck, Luigi gets brainwashed by Nastasia, turning him into Mr. L. He eventually becomes Super Dimentio, the fused form of Dimentio and Mr. L, which is the final enemy that the player fights in the game.
According to The InterNed, Mr. L's theme song is "Mr. L, Green Thunder".