“After another botched attempt to attack the Toppat organization, Henry Stickmin thought he couldn’t possibly have a chance to win. With him on the run, he couldn’t have expected to find himself at the doors of a tournament. Ellie and Charles certainly didn’t either, but they’re not above winning and seizing their prizes.”

Looks like not just Henry decided to turn up, but also his buddies. Coming from the same stickman dimension alluded to earlier, they're slightly different, as they're used to human-based machinery. Most of the results hadn't ended well for them, but they're used to it. Also, Ellie and Charles I'd imagine have the same movesets as Henry; they'd act as skins moreso than different character slots.