Luigi is Mario's little brother and also an hero.
Luigi first appeared in Mario Bros. as a second player who is ment to be Mario's brother. They where plumbers defending the sewers from creatures. He next appeared majorly in Wrecking Crew, serving as a construction worker who can. He later appeared in Super Mario Bros. as a hero in the Mushroom Kingdom, as a second player.
He had little personality developed besides his brother's until later on, where he was portrayed as someone who's scarred easiy and was heroic nonetheless. He was typically known to be away from a large amount of his brother's fame, living under his shadow, and manages to try and be heroic despite this.
He eventually became more brave when he saved Mario from ghost in Luigi's Mansion, where he had to push through ghost he was afraid of in a haunted mansion to save him.