Okay. I know the real truth of Luigi's existance in SM64.
1st - Go to the "Bowser in the Snow" course and get to Bowser without dying.
2nd - When you battle Bowser, make him flame you and run off the side of the battle arena.
3rd - When you die and end up back in the castle, enter "Fire Flamin' World" and beat the 2nd Big Bully next to the 3rd largest stone pillar.
4th - When you beat the Big Bully, jump into the lava and kill yourself.
5th - When you reappear in the castle, run outside into the courtyard and kill all the Boos out there.
6th - Luigi's hat should appear near the water fountain. Collect it and you will turn into Luigi!
See, now that's how you find Luigi in SM64!
*Note*: I know that Luigi is NOT in SM64. You guys are all full of crap! I repeat, LUIGI IS NOT IN SM64!