Use the cheat codes at the character selection screen or while playing the game to unlock the corresponding effects:
All characters: Right, Left(2), Right, Up(3), Start
Invincibility: Down, Up, Down, Up, Right, Down, Right, Left, Start
Level 99 characters: Up, Down, Up, Down, Left, Up, Left, Right, Start
Add 1point to all skills: Left, Right, Left, Right, Down, Up, Start
Extra melee damage: Left(2), Right, Left, Right, Up, Start
Fast characters: Up(3), Down, Up, Down, Start
Infinite Xtreme tokens: Left, Down, Right, Down, Up(2), Down, Up, Start