After Luigi was captured by Count Bleck at the beginning of the game, he was brainwashed by Nastasia and was told to defeat Mario and his friends once and for all. Taking the alias of Mr. L, he was acquired a robot that he called Brobot because he had a strong, brotherly bond with it.
He encounters the heroes twice in the story, first at the end of Chapter 4 in the Whoa Zone and later in Chapter 6 after the Sammer's Kingdom is destroyed by the Void to become the World of Nothing. After being defeated by the heroes twice, Dimentio kills Mr. L who then reappears as his normal self (Luigi) in The Underwhere. Luigi has soon reunited with Mario again and joins him 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 and the Chaos Heart, becoming Super Dimentio. However, Super Dimentio is defeated, which freed Luigi and returned him to normal again.
Super Smash Bros.
In Super Smash Bros. for Wii U, Mr. L can be obtained as a trophy. In Super Smash Bros. Ultimate, Mr. L can be obtained as a primary attack-type spirit whose ability makes the user take extra damage from water attacks. His spirit battle is against Luigi on the Ω form of the Luigi's Mansion stage covered in fog, favoring special moves and his Final Smash meter charges faster.
From different points of view, Mr. L can be seen as both an upgrade and downgrade of Luigi. Unlike Luigi, he was very brave, confident, and adventurous of himself. He also seemed to have a very smart attitude, seeing as he built Brobot all by himself. However, Mr. L was extremely cocky, sarcastic, and overconfident, resulting in an egotistical and pompous behavior with many flashy poses, and all of Luigi's kindness and humbleness is non-existent. He also used any positive attributes that he gained from being brainwashed for evil deeds, which the real Luigi would never do.
In Super Smash Bros. Brawl, he is mentioned in Paper Luigi's trophy, but as Mister L instead.
His stats: Max HP is 40, Attack is 3 and Defense is 0.
In the Flopside Pit of a Hundred Trials, Shadoo copies all of the game's four main heroes. However, when he copies Luigi, he actually has Mr. L's silhouette instead of Luigi's.
Throughout the game, Mr. L is treated as an outsider by most of the minions, but Dimentio takes a special interest in him, aware of his power and manipulating him to serve his own evil plans.
Somehow, Mario and the others do not seem to recognize Luigi as Mr. L. When Mario and a restored Luigi return to Flipside from The Underwhere after Dimentio separates the former from his other allies, Tippi wonders if they had met before. Later on, Mario even tells Tippi he finds Mr. L familiar, but Tippi brushes it off. This is averted in the manga Super Mario Kun, where the heroes do not fall for this disguise, with Bowser even mocking him for it.