It's strongly implied to be Ariel (Body Corrupted) or Kumoko. But we don't know which for certain.
I would lean toward Kumoko because she's wanted to protect Sophia since they met, and she knows the reincarnators on a personal level, where Ariel's thought patterns are still Ariel's own, but with Kumoko mannerisms and ambition. Also, given that Kumoko knows that the elves have repeatedly attempted to kill Sophia's parents, she would be the one to break the news to Sophia and create that rather prominent hatred Sophia's has for them.
On the other hand, that fighting style seems more like Ariel's.