You need 32 star coins(not sine spirits!) and got to level design room go all the way to right and open the 2nd door and say hello to Luigi.
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enter the 32 Star-Coin door to toggle between Mario and luigi
You can’t play as luigi in Super Mario 64! Because he is not in the game
In the original you can play as Mario and Luigi. In New Super Mario Bros DS you can play as MArio AND LUIGI. IN NEW SUPER MARIO BROS WII ITS MARIO, LUIGI, BLUE TOAD, AND YELLOW TOAD.
Super Mario lives with his brother Luigi in The Mushroom Kingdom.
Luigi is not in Super Mario 64. But he would likely just be Mario with a green shirt, green hat, and an L on his hat.