Luigi
From Paper Wiki
Luigi is Mario's younger brother in the Mario series. 
Paper Mario
In this game, Mario and Luigi are invited to Peach's party at her castle. Before Bowser had taken over her castle Luigi managably escapes. For the rest of the game, he is at home begging Mario to go on the adventure with him. If Mario gains the Super Boots and sees a loose panel at his house, it is revealed that Luigi has a diary.
Paper Mario: The Thousand Year Door
While Mario saved Princess Peach and got the Crystal Stars in Paper Mario: The Thousand-Year Door, Luigi also had a mission of his own. After every chapter in the game, Mario can hear Luigi's story in Rogueport.
When telling Mario the story, Luigi exaggerates and tells a few lies to make the story better and make himself sound much more heroic, but Luigi's partners almost always secretly tell Mario what really happened. Later, Luigi gets interviewed and 5 Super Luigi books are sold.
Luigi's adventure involved rescuing Princess Éclair of the Waffle Kingdom from the Chestnut King. Luigi's story sounded extremely similar to Mario's in most aspects. He usually messed up during his adventure, such as accidentally knocking his partner Blooey the Blooper into lava.
Oddly enough he also appears in the audience when in battle. He also cheers Mario on during the final battle with the Shadow Queen. One of the word bubbles says "YOU GO BRO" in green letters.
Super Paper Mario
In the game Super Paper Mario was playable. He is noticeably braver here, rushing in to save Princess Peach, jumping on Bowser, and calling Count Bleck a goon. His special move is a jump that is so powerful it sends him out of the screen. He is the last character to be unlocked, joining Mario's party in the interlude between Chapters 6 and 7.
Brainwashed by Count Bleck's assistant, Nastasia, Luigi becomes Mr. L, also known as The Green Thunder. Before Mario could get his hands on the fifth Pure Heart, Mr. L made his dramatic appearance in the Whoa Zone. He attacks with a super jump and can use Shroom Shakes during battle. 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 Sammer Kingdom was destroyed. Here, he fights with Brobot L-Type. After being defeated, Dimentio betrays him and sends him to The Underwhere. He loses all memory of his having been 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 defeat Dimentio, but gets knocked out by Dimentio's suicidal attack. Princess Peach finds and awakens 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.
