Within the game's files are voice clips for five unused Create-a-Soul classes. These clips can only be heard in-game via mods or hacking. The unused classes are Noble, Scared Fighter, Beastmaster (Ranger), Clown, and Great Warrior.
Contributed by ShyanVixen
Within the games files exists four unused fighting styles. Two of them are prototype movesets for Lizardman and Revenant and the other two have no specific character attached to them. When hacked into the game, Lizardman's is only one that is usable, however it is incomplete and glitchy. The other three cannot be used and will cause the game to crash when the player is attacked.

One style has the character riding what appears to be an invisible horse. It is speculated that this style might have been meant for the Calvary unit in the Chronicle of Swords mode.
Contributed by ShyanVixen
Having a SoulCalibur III save file on your PS2 memory card can actually have devastating results if you move, modify, or delete anything on your PS2 memory card. These include:
• Chronicles of the Sword side-quest becoming corrupted.
• Your entire SoulCalibur III save file becoming corrupted.
• All data on your memory card becoming corrupted.
Contributed by Mr. Kite
Despite not seeming to be a true guest character, KOS-MOS from the Xenosaga series can be created in the Character Creation system. It's believed that the fact she must be made using the Character Creation system is a reference to her being an android, an artificial being that was created, not born.
Contributed by Psychospacecow