You can unlock three other characters. Here are their names and how to unlock them.

Gardenia: Deliver a letter to her at Theta: Soft, Solitary, Tri Pansy after reading the message on the board about her fan club.

Natsume: Get a Twin Blade weapon for her at Delta: Raging, Passionette, Melody.

Sanjuro: Get a Heavy Blade weapon for him at Delta: Hideous, Destroyer's, Far Thunder.