Luigi is Mario's taller, slimmer, younger twin brother. For many years Luigi had no role other than "player two" in the Mario games, save for a brief role in SMB2 when he played an expert jumper. Luigi's first starring role came in Mario is Missing (1993), an edutainment title that is now largely forgotten. He then appeared as a selectable character in numerous sports / racing titles, usually, like his brother, displaying "all-around" or average attributes. In 2001, Luigi fans finally got what they had been waiting for: Luigi's Mansion, the first true Mario game to directly star the plucky plumber.

Luigi has been described as more high-strung and jittery than his calm, heroic brother. He is also said to be somewhat less impulsive than Mario, and prefers to carefully think things through before jumping into the fray.