Apart from the seven characters already unlocked in the game, there are two additional characters that are unlocked as you progress in the story. The first such character that joins in the story is Sophia, who will join your party once you reach Shibuya Jail. You will not have to wait long for her to join the roster, as she joins the crew in the early stages of the Shibuya Jail.
The second unlockable character is Zenkichi Hasegawa, who joins the game in the later stages of the story when you are in the Kyoto Jail. Although you will spot Zenkichi Hasegawa pretty early in the gameplay, it is only after you reach Kyoto Jail that he joins the roster.