Luigi is a recurring character in the Mario franchise, who is none other than the main character's brother.
Fire Luigi is a transformation of the hero using a specific object : the fire flower. It is first introduced in Super Mario Bros. in 1985. It is subsequently found in other games such as, among others, Super Mario World and Super Mario Galaxy.
When in this form, Luigi gains the ability to throw fireballs and his outfit changes : his overalls turn green and his t-shirt and cap turn white.