Unlock Extra Hard Mode:
Enter the following at the Game Settings Screen: Hold Z and press L, L, A, and B.

Unlock Richie, Giovanni, Lorelie, and Bruno:
Select 2 player mode and when you get into the Player Select Screen on both controllers hold Z and push both L and R buttons at the same time. First you have to get the Key from the Spa Service by beating Giovanni.

Unlock Everything:
Go to the title screen hold Z and press A, B, R, A, A, B, R, A. This will enable everything, just look at the Puzzle University

Player 2 as Mewtwo For Player 2:
Play a 2 player game and on the Player Select Screen on controller 1 hold Z and push B, Up, L, B, A, Start, A, Up, R.

Levels 51-99:
In the Marathon Mode you can go up to the levels 51-99. On the start screen of the game (Where they show Ash with Bulbasaur,Squirtle, and Pikachu). Hold Z and push B, A, L