Luigi began as Mario's alternate. He looked exactly the same, except with da variation in color, the familiar green attire he wears today. ^_^ This continued until Super Mario Brothers 2, where Luigi finally gained a personality of his own, becoming taller and lankier and better then Mario in almost every way, ahehehe ^_^. God could he jump.
Anyway, sadly, back in Super Mario Brothers 3 and Super Mario World, Luigi returned to being a copy of Mario with different colors (cept fer da end of SMW, Luigi was shown bein his normal tall tall self. ^_^). Although he was the goal in Super Mario World 2, this was slightly different, rescuin him bein da objective.
Otherwise, Luigi has always been da second Mario brother, sometimes not even showin up at all (Super Mario RPG (kinda) and Super Mario 64, fer example). He did star in his own game, Mario is Missing, where he was da hero, altho he STILL essentially was a copy of Mario. ^_^o He makes occasional guest appearances at times or is a secret character (Super Smash Brothers) but has never really been in the limelight...until recently.
Luigi's Mansion is da new game fer da Gamecube where Luigi is da star and has ta rescue Mario fer a change (about frickin time if ya ask me...)...I'm gonna be playin it soon, so there'll be more about that as I learn more. ^_^ But finally Luigi is gettin da recognition he deserves!
But other then that, Luigi has almost always been in the background, waiting...
Luigi is taller and thinner then his older brother, Mario, and, or so it says in da Mario Party manual, is smarter as well. On the whole, however, Luigi seems more introverted then Mario. He's quiet, shy, and doesn't like attention that much. He's more laidback and accepting then Mario is, but not nearly as confident. He also doesn't seem very confrontational, and probably prefers remaining in the background, unobtrusive. He doesn't like to confront people about things or how he feels and is far too shy or unsure of himself to speak his mind...
Or so I think. ^_^