Play as Ambia:
Defeat the three opponents in the first tournament in tournament mode to unlock an amoeba player which turns into a racquet when hitting the ball.

Play as Kaoru:
Defeat your opponents for the Aries cup in tournament mode to unlock a fat player.

Alternate costumes:
Highlight a character on the selection screen, then press B.

Play with the ball:
Press Down before serving, to bounce, toss, or do something else the ball (depending on your character). There is only one animation per character, but most characters' animations are unique.