She could be Asta's older sister because she can't have children with anyone because of her condition. Asta's mutation could be a result of physical contact between them.
Well, they do look alike, both in appearance and personality.
Maybe Asta is the manifestation of Liebe's (Anti-Magic Devil) humanity. Maybe once a devil develops a conscience as a result of bonding with a human, their humanity manifests in a form of a human version of itself. Once the women...
The Anti-Magic Devil is having a flashback involving Asta's mom. Is it possible that the Anti-Magic devil was a human before turning into a devil? If so, what is his personal connection to Asta's mom and Asta himself?
It will probably give us a hint that Asta will enter the Other World if he were permanently exiled from the Clover Kingdom. Even if he clears his name and saves the Clover Kingdom again, it still doesn't take away from the fact that the Clover Kingdom cannot trust someone with demonic powers...
The woman in the bed with Dante in the flashback... She has light hair and she seems to have a cowlick. Sounds familiar? Could it be Asta's mother? If it were true and if Dante slept with her, then he is her lover and possibly Asta's biological father.