New Super Luigi U
Play as Nabbit
Hold ZL when selecting a level in single player mode. Note: Luigi will return as the main character after the level or if Nabbit dies.
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Play as your Mii
Hold ZL when selecting the final Star Road level in single player mode.
New Super Mario Bros U movements
Successfully complete the game. You can hit the red "M" block that now appears at the start of each level to make Luigi move like Mario in New Super Mario Bros U.
Strategy guides from GameFAQs