So then who is her real father?
It is still Athanasius, and I believe Athanasius' appearance is exactly like that black haired dude.
If you don't mind novel spoilers,
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Athanasius' character shouldn't have any more reappearance after his story with Claude was told. He was just dead. In the novel, all the things this black haired dude did was done by a character named Carax, who was Aeternitas in a random person's body. Aeternitas was one of Obelia's king who longed for Lucas' recognition & loyalty but never got it. He engaged in multiple black magic practices to keep being alive to get back to Lucas someday.
And mind you, Jeannette was a product of black magic anyways. She was not normal and technically speaking she has no real parents. Athanasius & Claude's ex-fiancee made Jeannette together with black magic.
If this webtoon is still remotely similar to the novel,
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A Bit of Spoiler
That black haired dude is not really Claude's brother but merely the Lucas-obsessed wizard who had to use tons of black magic to keep being alive.
So him being a douchebag & using Jeannette for his own gain is not far fetched, because he is no one's father.
But if this webtoon still have the audacity to change even that part of the plot, then lol go crazy, I won't expect anything anymore