It could have been Jeanette, she's an artificial child created by the previous emperor (Claude's brother) with dark magic. So she's essentially a doll or puppet, and her original purpose was to harm Claude, so what better way to bring him down than to have her (Jeannette) constantly hug and touch Claude (in the original story) and slowly fill him with that dark magic without him knowing or suspecting the girl (although he knew she wasn't his daughter).
It could also be the son (Ezekiel or something) he's the male lead, but he can also be used and manipulated to pass on dark magic or matter on to victims. He also has shifty eyes, I don't trust him (could be wrong of course)
It is certain that the black haired dude (speculations that he's actually the previous emperor and it would fit) is responsible for the dark magic, only question is through what medium is he attacking Athy.
So Lucas clearly had to remove some dark magic from Athy, which led him to believe she had danced with someone who was trying to harm her. But the only people we saw dancing with Athy were her father, Felix (both would never hurt her), the other noble girls at the group dance, and Mr. White's son. Maybe you can count Jeanette giving back the ribbon too. So who tried to curse Athy?