Oh right. I hadn't been paying attention and thought he was just some generic handsome guy, but he's from that mage faction (that we saw talking in chapter 56) and came to see the reaper for himself. I really do wonder how they manage to infiltrate this fast and just get invited to a high profile party like this (wasn't this organised by the king?). If they can do that, this kingdom's over.
It doesn't seem that Olivia sees him as a threat, but they'll be dancing, so I wonder what'll happen. He has no clue how much danger he's in, since she can just grab his heart if she suspects anything. Well, the truth is that even if she suspects something, she'll care more about getting back to the food than causing a mess that might spoil the mood and prevent her from getting back to the food. She's also confident in her ability to take him out if they meet on the battlefield, so if he's just here to scout and she doesn't have orders, she probably won't act.