Successfully complete Story mode.
Hold L + R then press Up, Down, Right, Left, X, B, Y, A at the title screen. The sound of a bell will confirm correct code entry. All bonuses including characters and a completed gallery will be unlocked.
Successfully complete Story mode. Play Act 3, Stage 1 again with Amitie. In the intermission sequence, Amitie wants to join Draco Centauros' beauty contest and hears something in the bushes. Harpy and Witch from the original Puyo Puyo will appear from the bushes.
To unlock everything including characters and filled up gallery enter the follow commands at the title screen. You will hear a bell chime if it was done correctly.
This is how to unlock Risukuma.