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