Mr. L is an antagonist in Super Paper Mario. He's the evil alter-ego of Luigi. He is a boss in World 4 and World 6 in Super Paper Mario. He was originally Luigi, but he later got brainwashed by Count Bleck and he was told to defeat Mario and his friends once and for all. He also has a robot that he calls Brobot. Later in Super Paper Mario he appears as his normal self in Underwear. He was reunited again by Mario to help his friends out.
He also technically serves as the final boss, as Dimentio brainwashes Luigi during the final battle, then fuses with him. However, Super Dimentio is defeated, and the day is saved.