You could choose who to keep in control of body. And on nathema, other come to join conspiracy. Those who choose Khem, get him back in onslaught. Those who choose Zash, however, did not. But as Nathema conspiracy show, that she is capable to get new body, i expect her back. Other way SI is only class that possibly never get 5th companion back, depending on choice. Which is, unfair, and as said above, don't make sense.