I was thinking recently and something occurred to me. There is a good reason why Soma may have been omitted from Judgment. For those who may not remember, when Soma is in a dangerous situation his "power of dominance" awakens allowing him to absorb the souls of the monsters he slays and thus gains their powers. However, when the situation is resolved, the souls are released and he loses the abilities he absorbed.
Because of this, Soma could have been omitted. After all, he would have any powers. The only way it would work is if he was transported to the area the game takes place in during either of his adventures. Considering that this is unlikely, I doubt Soma could work as a character for Castlevania Judgment.