Acela Benkan Salvatore is known as the Monster Duchess as well as a powerful swordsman and magician. She quickly adopts Leslie after seeing the power of the Black Saint.
She is Sairaine's wife.

The Speràdo family line possesses a secret: shadow magic. But it’s been 100 years since someone last wielded it. When Marquis Speràdo tries to sacrifice Leslie for her favored sister Ellie, little does he know that this awakens the power of darkness in her instead. To escape her family’s greed and abuse, Leslie’s out to make a deal with the Monstrous Duke: adopt her, and her powers will be at the duke’s disposal. Will Leslie escape her parents’ cruel grip, or succumb to their evil exploits?
 
(Source: Tapas)
 
Note: Chapter count includes 20 extra stories.