Baby Luigi is Luigi as a baby. He is Baby Mario's brother, and Baby Daisy's boyfriend. He is a small class driver, meaning he can only use small vehicles. Also, his clothes are similar to Baby Mario's, only green. He is an unlockable lightweight character. He, Luigi, Mario, and Baby Mario are the main characters of Mario and Luigi: Partners in Time. He is also one of the main characters in the Super Mario World series. He is also in several Mario spinoffs.
Super Mario World series
Baby Luigi did not appear in Super Mario World. However, he was one of the main characters in Super Mario World 2: Yoshi's Island. In it, he gets kidnapped by Kamek, and Baby Mario and the Yoshis try to rescue him and bring them to their parents. In Yoshi's Island DS, Baby Luigi is one of the Star Children, and after he gets kidnpped by Kamek, Baby Mario, Baby Peach, Baby Wario, Baby Donkey Kong, and Baby Bowser try to save him. He never finds out that he is a Star Child, too. In Yoshi's New Island, Baby Mario and Baby Luigi get delivered to the wrong parents, and Baby Luigi gets kidnapped. Baby Mario and the Yoshis save him, and the Stork delivers them to the right parents.
Yoshi Touch and Go
Mario Kart series
Mario Kart: Double Dash!! is the first Mario Kart game that Baby Luigi appears in. He is a playable light-weight character. His default partner is Baby Mario, and his default kart is the Rattle Buggy. Their personal item is the Chain Chomp. Their personal course is Baby Park. In Mario Kart Wii, he is a light-weight unlockable racer. He has high acceleration, handling, drifting, and Mini-Turbo, but low speed and weight. He can be unlocked by unlocking eight Expert Staff Ghost Data in Time Trials, winning 100 WFC Ghost Races, or playing 3,150 races. In Mario Kart 8, he is a starter lightweight playable character. He has good acceleration, handling, grip, but he has bad speed and weight.
Mario Baseball series
In Mario Superstar Baseball, he is an unlockable speed character. He can be unlocked by completing Challenge Mode using Yoshi's Team. He is great at stealing bases and fielding. His special ability is the Wall Jump. His batting and fielding stats are low, and he has average pitching. In Mario Super Sluggers, he is a starter character. In it, he has better fielding skills, and his special ability is Super Jump.
Mario and Luigi series
The only Mario and Luigi game that Baby Luigi was in was Mario and Luigi: Partners in Time. It is the only game that he is one of the main characters in, along with Mario, Luigi, and Baby Mario. He will help Mario, Luigi, and Baby Mario attack enemies. He rides piggyback on Luigi for most of the game, but will get off of him to attack enemies.