From the Super Mario Wiki, the Mario encyclopedia
Better Together is a level in Bowser's Castle (West) in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
Bowser Jr. and the Koopalings find Roy on the doorstep of Bowser's Castle, exhausted from fighting so many Malatone-infected minions by himself. Roy still wanted to fight them, however Bowser Jr. picked a fight with him as a way to stop him exhausting himself more. Kamek heals Roy, knowing that a fight between them won't be prevented and hoping the fight will end in a draw if they are both at full strength. After the fight, Roy rejoins Bowser Jr.'s team.
During the level when played for the first time, Bowser Jr. is set as a flying type and has no troops with him at all, not even a First Officer. To compensate this, Roy has no troops either.