With an Adult Tamagotchi, win ANY tournament. You will get a symbol above your tamagotchi's picture on it's stats screen. Now, go to the options menu. There will be a mode called MUSIC MODE. Inside are:

Sound Test
Sound FX test
Sound Test Speed
A gallery of ALL the characters you get after that Tamagotchi wins. They are numbered and you have to get all of them to beat the entire game.