Luigi Cap
Luigi 's Hat has been a key symbol for the Super Mario Bros. franchise. Luigi has had his hat in hand for his every appearance with no exceptions. Luigi has worn the hat since he was a baby.
History
Luigi's hat was meant to contradict Mario's red hat. Green is the Negative Color of Red. The hat was made green with a green "L" emblem. It has stayed that way ever since.
During the events of Super Mario World, the tip of the cap had been changed to Black instead of his usual green, this happened for Mario as well.
Importance
In Super Mario 64 DS Luigi's hat was used to shape-shift into Luigi if the player was either Mario, Wario, or Yoshi. In the same game, Luigi is effected the same way as Mario and Wario if their hats are lost. Luigi's defense and attack sharply decline while his hat is absent from his head. If the player gets the hat back, Luigi's attack and defense revert back to normal.