Mr. L is actually Luigi dressed up like a villain in the Wii game Super Paper Mario. He takes orders from Count Bleck. Mr. L has a robot called Brobot which Mario and his team will also have fight in the game. Near the end of the game, Dimentio betrays him and puts a floro sprout on Luigi to control him. Mr. L (now as a brainwashed Luigi) is used to form Super Dimentio, along with the Chaos Heart, and of course, Dimentio himself.
It is strange how no one (except Count Bleck and his minions) could identify Mr. L as Luigi, since his green cap, voice and mustache give it away.