Luigi

Luigi (full name Luigi the Sexy Plumber, also known as Luigi Gaywalker) is Mario's younger twin brother.

Personality
Luigi is kinder and more understanding than most of the other characters. He is often cheery, enthusiastic, fun, and ready to lend a helping hand. His main weakness is his cowardice, and he gets very nervous when doing something dangerous or immoral. On the rare instances when he is angry, he can be very unrelenting and mean, though he usually reverts back to his normal personality when he calms down.

Luigi has often been accused of being gay. He usually denies this, but he has shown homosexual behavior many times. He was attracted to Daisy in The Adventures of Mario and Luigi - Ep. 4, Awkward Weddings, so he may be bisexual.

Mario
Mario heavily abuses his brother/sister Luigi, often insulting him, beating him up, and in early bloopers killing him for no reason, although he always comes back to life for some reason. Why Mario does this is unknown, but it may be because he resents Luigi for outperforming him; Luigi always beats him in Super Smash Bros, Mario Kart and Mario Party, and Mario gets very angry when Luigi out-jumps him in Long jump. Luigi also gets angry at Mario when he is being too stupid.

Despite their rivalry, Mario and Luigi do seem to truly care for each other. This is especially apparent in Luigi's Retarded Mansion, when Luigi risks his life to rescue Mario.

Mario and Luigi's true relation is unknown. While they are usually said to be brothers (which is true in the official Mario canon), there are two conflicting account of Mario and Luigi's relation. In How Mario was Born, Mario gave birth to Luigi, and in The (TOTALLY ACCURATE) documentary of Mario, the two met at an orphanage.

Toad
Toad does not like Luigi, and is condescending towards him. He has said "No one wants to hear your shit, Luigi" multiple times.

Daisy
Luigi is in love with Daisy, and tried to romance her in The Adventures of Mario and Luigi - Ep. 4 and Awkward Weddings. Luigi claims Daisy doesn't like him; whether this is true or Luigi just thinks it is is unknown.

Waluigi
While Waluigi is Luigi's self-proclaimed rival, he has yet to be seen directly antagonizing him in SMG4's bloopers.

Trivia

 * In "Crystal Funhouse", Luigi is called "Luigi Gaywalker", but it is likely that "Gaywalker" is referring to his role in Mario's Star Wars battle as opposed to being his actual surname. The line "No! I like being Gaywalker!" further supports this possibility.
 * Luigi owns a Nintendo 64 and a Nintendo 64DD.
 * He apparently has a PhD, as shown in "Retarded64: A Dose of Dr. Mario"
 * It's revealed in "Operation G.A.Y" that he wears female underwear (Princess Peach dress).
 * In "Guards N' Retards: Prisoners" and in "SwagQuest", Luigi is seen driving a kart and making an angry face at the prisoners (to Mario in Swag Quest). This a reference to his "Death Stare" from Mario Kart 8, which is very popular on the Internet. The Music playing while luigi doing it is "They see me rollin'" of Chamilionaire.
 * It was revealed in "How Mario was born" that Mario actually pooped Luigi out, meaning that Mario is technically Luigi's mom but in "The (TOTALLY ACCURATE) documentary of Mario" the two meet in an orphanage.
 * On some bloopers the characters says that Luigi is a girl (Though this might be a lie) like in "Ssenmodnar 5 (WAT (shocked face) Edition)" one of the facts about Mario says that Luigi is actually Mario's sister and in Bowser and the nitghtmare stone Bowser says to Smg3 I already zapped Mario and his sister (Luigi).
 * In SwagQuest, Luigi said that he was in love with Daisy.