If the Final Boss is beaten with Nabbit, he'll make a small cameo as Luigi goes up to Princess Peach, spying on the two of them.
It's Possible to play as Nabbit in any level during Single Player. This is done by holding down the ZL Button on the Wii U Gamepad or Pro Controller, or the B Button on the Wii Remote.
The development team originally considered expanding the Boost Rush mode in New Super Mario Bros. U, like how they expanded the Coin Rush mode in New Super Mario Bros. 2. However, they felt that they should come up with a new idea, rather that do something so similar. The team began experimenting with short, difficult levels, intended for advanced players. This led to an entirely new game with more challenging courses which became New Super Luigi U.
Nintendo originally planned to have a third Toad character to replace Mario, but testers found it too confusing with three Toads on screen at once. Nabbit was later added, as his ability to pass through enemies would be helpful for new players.
By pressing the ZL button on the Gamepad or Wii U Pro Controller, or the B button on the Wii remote before entering the level Flying Squirrel Ovation, you will be able to play as the Mii assigned to that account.
There is a hidden Luigi image in every level, totaling 82 different locations.