perhaps Felix' niece he talked about is actually his daughter from an affair and the reason why we have never seen this girl, who is supposedly Athy's age, is because we have already met her! it's the purple haired girl or the brunette girl whose mother Felix dated years ago.
Felix couldn't have talked freely about his child because sex before marriage that resulted in a bastard could cause a scandal. His previous girlfriend had raised the girl as her husband's legitimate child and she would lose everything if her husband found out about the truth. That's why he changes his daughter to his niece whenever he talks about her, to protect her. This would make Felix in a way like an inverse of Claude. He might have been more involved in his daughter's childhood but grew distant when she became older. The girl truly believes she's the daughter of her mother's husband wheras Felix knows he is her father but is forced to remain silent and guard his girlfriend's secret.
