YA fantasy or science fiction book read in 90s with demon hero who can use an illusion to appear human

[Louis]

The hero is a demon who has a sort of mask illusion that makes him look human. I can only remember one scene where he and his group enter a tournament of some kind (against aliens?). The aliens are worried they might not be up for the tournament, so they drop their illusion. This normally frightens others, but the aliens are surprisingly relieved that this will make a fair contest. The alien 'football' team are not the normal aliens the heroes have seen, but great beasts. I was reading a lot of Terry Pratchett at the time, so expect it was something in a similar vein, maybe younger YA fantasy.