It is insane how 100% of Luigi's personality comes from Luigi's Mansion. Before that, he was just Green Mario. Like, they radically changed Luigi's entire personality for their comedy-horror game, and then they actually stuck by that. It's like the only piece of continuity in the entire series, is Luigi being a coward. Everything else shifts constantly EXCEPT his personality.
Luigis Mansion, Smash Melee, and Superstar Saga all released pretty close to each other and all really differentiated Luigi. I think thats what caused it to stick. Some props should go to Paper Mario too with Luigis secret diary.
WarioWare