There are two other characters that you can unlock. Here are their names and how to unlock them.

Balmung: He is able to join your party after a Net Slum event.

Terajima Ryoko: Read her message on the board. She is stuck at the bottom of Sigma: Cracked, Worst, Milestone. There are five floors on this dungeon. NOTE: Terajima wants to explore a level 62 area after saving her. She is only level one, so I would suggest leveling her up.