How do you unlock luigi in Mario 64 ds?

After unlocking the red ! boxes, go to the castle courtyard. Punch the giant boo with the cage inside of him to enter Big Boo's Haunt. Obtain the first star by defeating all the Boos to get the staircase. In the second goal, as Mario, go up the stars and into the room with the single bookshelf. Use the fire flower to float up to the room above, where you'll find a Luigi painting. Jump through the painting and defeat Big Boo to get Luigi's key.