There's data within the game's files for two unused characters, Wovama and Q.Takagi. When hacked into the game, both characters use Cassandra's model and will randomly pick a weapon and moveset. While the model and movesets for both character's were never created, their names do appear on the verses screen. But when used in game, Wovama's name remains blank under her health meter, and Q.Takagi's name will change to Spawn. Because neither character have models, it is speculated that they were cut very early in the game's development.
Contributed by ShyanVixen
In a episode of the 8-4 Play podcast, localization producer Nao Higo confirmed that Cloud Strife from Final Fantasy VII was going to be the guest character of the Playstation 2 port. However, the deal got canned at the last minute, and Heihachi from Tekken replaced him in the end.
Contributed by ZpaceJ0ck0
Each of the characters that didn't appear in the arcade release of the game are paired up against one of the four newcomers for their destined battle.

* Heihachi/Spawn/Link are paired against Raphael
* Necrid is paired against Talim
* Sophitia is paired against Cassandra
* Seong Mi-na is paired against Yun-seong

The rest are mutual battles between arcade version characters.
Contributed by AkiraTheWarrior
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Soul Calibur II was the first game in the series to be a sponsor of a wrestling event, WWE Unforgiven 2003.
Contributed by GorgonBull35