i think only human can take the sword out... Since InnaYashua's mother is a human being, and his father realized how weak and fragile human being is compare to monsters. I think his father just assumed that InnaYashua won't need the sword THAT much if InnaYashua has a monster lover ba~
(which means.... I think InnaYashua is weaker than his father thought he would be..... he used the sword to save himself more than saving Kogma.... and sometimes he actually needs Kogma to save him :P....... just my thought, for those people who love InnaYashua, don't kill me okay? :D)