In many ways, Luigi is a foil to his brother Mario. Though Luigi is heroic and kind like his brother, he has a tendency to chicken out of some situations that seem to be too scary or beyond his capabilities; he may even cry if something were to upset him dearly. Luigi desires to be like his brother Mario and also become a hero of the Mushroom Kingdom. In the Mario and Luigi games, Luigi would usually try to back away from boss battles, leaving Mario to encourage him into helping him fight. But when he needs to, Luigi is as brave as Mario during events with strong enemies. He seems to be afraid of ghosts, and startled quite easily as seen in the Luigi's Mansion games.

Charles Martinet has stated in various interviews that he invented Luigi's voice, and his current personality, on the spot, while doing "Mario In Real Time". Children would ask him where his "brother Luigi" is. At the time, his interpretation of Luigi had not been conceived, but he improvised Luigi calling out that he's in the kitchen, and that he doesn't want to come out because he's too shy, in what would go on to become his trademark "timid" sounding voice. This was in-keeping with how Luigi had been portrayed prior to this, as somewhat of a coward, and helped to cement this personality for the character.