I suggested that the player could be a Cerebrate created by Kerrigan to ease her workload, with the caveat of a mental control that prevents it from rebelling against her. It's really as simple as that.
Or they could be a NPC race, as Kaz suggested.
Oh, and I think reviving the Toss Purifier/ Soul Hunter concept would be a cool idea. You could have the bolt of the Lightning Gun be capable of restoring the health of allies nearby or drain the health of enemies to heal the user.