Unlock all characters in Hot Shots Tennis for the PlayStation

Me and three of my colleagues have been playing Hot Shots Tennis for the PS2 for endless hours during our lunch breaks at work (at a Game company).

I left the company in the mean time, but to celebrate E3 we were planning for a reunion game. We made sure that we could borrow an old PlayStation 2 for tonight with 4 player multitap, but we forgot that we have to unlock the characters again!!

Of course is is pretty essential that we can play with Will to keep the quality of the games high!

Are there any cheats to unlock all the characters in Hot Shots Tennis - without spending hours on singleplayer mode. Stuff like Triangle Triangle Circle Square Square Left Right to instantly unlock all characters would be great. Any other hints how we can unlock Will faster would also be really appreciated.

(The game is in Europe also known as Everybody's Tennis)