The first in-depth reveal trailer for the Bowser's Fury mode in Super Mario 3D World + Bowser's Fury revealed that Bowser Jr. is joining Mario for the adventure. Bowser Jr. can help you out by fighting off some enemies and dropping in power-ups for you. A second player can also jump in and take control of Bowser Jr..
If you're someone who doesn't like help when you're playing a game, you'll be happy to know that you can adjust Bowser Jr.'s settings. There are various levels of support to choose from, which will have Bowser Jr. hanging at your side most of the time, or barely offering up any help.