Mr. L, also known as The Green Thunder, is Luigi, brainwashed by Count Bleck's assistant, Nastasia. He attacks with a super jump and can use healing items during battle. Before Mario could get his hands on a pure heart, Mr. L made his dramatic appearance in the Whoa Zone (complete with stylish poses). When he is bested in combat, he reveals his trusted giant-robot "brother", Brobot, which he attacks Mario with in space. When he was defeated, he vowed to return. Later, at Count Bleck's lair, Mr. L did not seem pleased with Bleck's minions, going so far as to call them "junior" minions and calling them weak.
He returned as a boss in Chapter 6, after the dimension was destroyed. Here, he fights with Brobot L-Type. After being defeated, Dimentio betrays him and sends him to the Underwhere. He loses memory of him being Mr. L and turns to normal. Mario finds him scared in The Underwhere, and Luigi joins Mario on his quest.
Eventually, he faces Dimentio in Castle Bleck alone, not even with a single Pixl by his side. Nevertheless, he manages to easily defeat Dimentio, but gets knocked out by Dimentio's suicidal attack. Peach finds him and awakes him. He later rejoins Mario with Peach and Bowser and the four heroes' power makes Count Bleck defeatable. With Luigi's help, the heroes defeated Count Bleck and saved the universe. However, Dimentio snatched the Chaos Heart, saying that it was Mr. L's turn to shine once more. Suddenly, a brainwashing sprout emerges from Luigi's head and puts him under Dimentio's control. Dimentio says the Dark Prognosticus states that he is the chosen one to absorb the Chaos Heart. Luigi fuses with the Chaos Heart and Dimentio to become Super Dimentio. The other three heroes defeated Super Dimentio, and turned Luigi back to normal. Luigi then witnessed Lord Blumiere and Lady Timpani use their love to stop The Void.
Info from http://www.mariowiki.com/
