Loved this game! Gave me nostalgia to some of the games on NG when I was a kid. I'm a tennis fan so this might not have been tennis but it was close enough! I was excited to play the game just from the tutorial but you made every match genuinely engaging, and some even challenging. I'd recommend adding some achievements though.
SPOILER ALERT
Some strategies I had:
Momoko - Serve the ball, run to the front of the court, hit towards center, her clone will defend in the center, turn into a log, then just return the ball towards whatever side the real momoko isn't on.
Hector - Serve the ball, run to the front, return the ball ASAP to whatever side Hector isn't on, his second ball won't even have time to score on you.
The twins (did they have names?) - I found that the only spot they couldn't really return the ball on was if you hit it towards the center.