Luigi is Mario's heroic sidekick and brother. He has fought alongside Mario on several occasions. He shoots green fireballs instead of red, like his brother. He made an appearance in all Super Smash Bros., and being unlockable in the original for the N64. Since most Mario games nowadays have gotten rid of two player, Luigi has obtained his own personality and identity. Luigi appears in a number of different games, most notably these:Super Smash Bros.
Super Mario Bros. 1, 2, and 3.
Super Mario World
Paper Mario: The Thousand Year Door
Super Mario: The Legend of the Seven Stars
Mario and Luigi: Superstar Saga
Bowser's Inside Story.
and Super Paper Mario. Including many others.