Unlock Mario Key -

Beat Goomboss behind 8-Star Door in the Rec-Room   

Unlock White Room Key -

Catch all the Shining Rabbits   

Unlock Wario Key -

With Luigi in Mirror Room, hop into Wario's Painting and beat the Giant Ice Bully  

Unlock Luigi Key -

Beat King Boo in Luigi's Painting on the 3rd Floor   of Big Boo's Haunt

Submitted by Fi