Boo usually gets left out of the Mario spinoff games, but with a massive roster, he's certainly here for Mario Tennis Aces.
A few players online already gave me the heads up on Boo's specialty. He's a tricky tennis player.
Which means his tennis balls curve, often very unpredictable too. Which is good for when you use him, but very bad if you're up against him.
For which I think he's the best of his class. And overall one of the better characters in the whole game.