Enter these buttons at the main menu screen. The character unlocked will speak to confirm the code

Code - Effect

Square, Circle, Triangle, Right, Down, Up, X, Square - Unlock Kamen Rider Dark Kabuto
Triangle, Left, Circle, Circle, Down, Up, Square, X - Unlock Kamen Rider Hyper Gatack
X, Up, Triangle, Square, X, Triangle, Square, Down - Unlock Kamen Rider Hyper Kabuto