On normal level or higher, before fighting Clair, defeat your opponent in less than 25 seconds to face Lass. Defeat her and you'll get Bellossom.
Debug Menu
At the Title Screen, press Up(2x), Right(4x), Down, Left(10x), Up(4x), Right, Down(6x), B. At the new screen, press the A button until you hear a sound effect then press the B button until you hear a sound effect.
Disable Stops
When you power on the game, wait until Pikachu smiles, then enter: Left, A, B, Up. With this code, the blocks will no longer temporarily stop rising when you score a chain or combo.
Igglybuff
Complete Challenge on Easy.
Magby
Score 100,000 or more in Garbage mode
Marill
Make a 4-chain before beating Clair on normal or higher level and you'll fight a swimmer. Defeat him to get his Marill
Max. Speed Increase
At the beginning, right when Pikachu smiles, enter: B, A, Left, Left. This will increase the speed drastically if you're on speed 99. Normally, anything at or above 50 will continue at speed 50.
Pichu
Play challenge mode on normal or harder. Last for at least 4 minutes on any round before reaching Clair. A Pokefan should challenge you. Defeat her to get Pichu.
Pikachu
Make a 6-block combo before fighting Clair on normal or higher level and you'll fight a Gentleman. Defeat him to get his Pikachu.
Sentret
Make a 7-chain before fighting Clair on normal or higher level and you'll fight Beauty. Defeat her to get her Sentret
Togepi
Make a 10-block combo before fighting Clair on normal or higher level and you'll fight a Pokefan. Defeat him to get his Togepi.