A.I. partners are determined by who you have resurrected. If you resurrected 10 bones and they ended up being 1 Sorceress, 4 Dwarves, and 5 Fighters, then you have a 5/10 chance of getting a Fighter in one of your party slots, 4/10 chance for a Dwarf, and 1/10 for a Sorceress. Check your party list in the tavern, you probably have few to no Sorceresses and plenty of the other classes. You can always dismiss low level A.I. characters or multiples of a class. As for online with other human players, maybe no one you played with uses a Sorceress.