Howlies FUPAsaco,
The creator of Tennis Ace is a member of our Discord. Since we all enjoy making games at the same time, we've also been playing around with cameo appearances of numerous sorts appearing in each other's games. See if you can spot them all when playing "Extracurricular Activities", "Tennis Ace" and "Sins and Paradise".
P.S There's a high chance that you'll see Sins characters pop up in other game titles in the future.