In Quest mode when you have Ivy, Taki, Astaroth, Mitzurugi, and Sophitia in Chapter 3, you will see a scene featuring Lloyd Irving from Tales Of Symphonia. When the scene ends, a fight will start. After some time, it will stop, and a new fight will begin. If you win, Lloyd Irving will join your group.

Successfully complete the game to use any character in any mission.

Successfully complete the quests that are given in Chapter 5 to unlock your character's third weapon.

Earn the "Paladin", "Master", and final rank in your quest to unlock your character's fourth weapon.

Successfully complete the indicated task to unlock the corresponding weapon:

Astaroth Alpha

Great Maul: Earn the "Diamond Paladin" title.
Terror Moon: Successfully complete the "Return Of Guardian Ammon" quest.

Ivy

Black Widow: Earn the "Garnet Paladin" title.
Valentine: Successfully complete the "Imperial Army Headquarters" quest.

Lloyd

Angel's Tear: Earn the "Edgemaster" title.
Shiden: Successfully complete the "Guardian Cervantes' Revenge" quest.

Mitsurugi

Pandemonium: Earn the "Ultimate Master" title.
Shishi-Oh: Successfully complete the "Defense Line In The Wilderness" quest.

Siegfried

Requiem: Successfully complete the "Seal Soul Edge's Power" quest.
Soul Calibur: Earn the "Legendary Hero" title.
Soul Edge Omega: Defeat Guardians Fafnir, Ammon, Geki and Maki, and Cervantes.

Sophitia

Hyperion: Earn the "Ruby Paladin" title.
Omega Sword: Successfully complete the "Beyond The White Darkness" quest.

Taki

Fu-Ma Kugi: Earn the "Sword Master" title.
Mekki-Maru: Successfully complete the "Return Of Guardians Geki and Maki" quest.

Successfully complete the game to unlock all alternate costume colors for all characters.