I'm the Villainess, So I'm Taming the Final Boss

Aileen finds out that Claude's aide, Keith Eigrid, has been selling demons. Keith tells Aileen about the reason why and reminisces about their past. Now that Aileen knows his true feelings, she decides to cooperate with Keith to prevent Claude from despairing about humanity and turning into a demon. At the same time, Cédric and Lilia visits the demon lord's castle to tell Claude Keith's secret?!

Aileen overcame all the difficulties and got engaged with Claude. Just when she thought she found her happily ever after, she receives news that a demon named Ashtarte attacked a human village in the Mirchetta region. Sensing new dangers, Aileen infiltrates Mische Academy in the Grand Duchy of Mirchetta to investigate the situation. There, she meets James Charles, the last boss of Regalia of Saints, Demons, and Maidens 2 as well as the other quirky members of the student council.

James turns into a demon at a different point from the original game, but Aileen decides he can still revert back and lets him escape. However, the student council member Walt Lizanis witnesses the scene and begins to doubt Aileen's identity. In turn, she exposes Walt and Kyle Elford's true identities and their mission. They decide to temporarily work together to solve the vampire case. Meanwhile, Mische Academy receives another statement from Ashtarte...

"James is suspicious of Aileen since she knows of his demon form. However, Ribbon's existence and Aileen's words ""I want to protect you"" softens his heart and he decides to work with her on the case. The day of the school festival finally arrives and Aileen patrols the area with James in duck costumes. Meanwhile, Claude tosses his work to the side and goes to the school festival to have some fun. He meets a certain duck again when he entering the swordsmanship tournament."

Knowing that the prize of the contest for White Lily Princess is demon incense that's harmful to demons, Aileen removes her male student disguise and enters the contest dressed as a woman. While others are overwhelmed by her beauty, James asks her to dance. The light dance steps of the pair fascinates the audience and Aileen is chosen to be the White Lily Princess. However, right after that, the mastermind has demon incense scattered throughout the venue. Aileen and the others struggle in vain as James turns into a demon.

Claude loses his memory and magical powers, returning him into a normal human being. In order to show the appeal of that aspect to the public, he goes on an inspection of the third zone of the imperial capital. During this time, Aileen follows Claude and tries to confirm her position as his fiancée. However, she just ends up experiencing the bitter feelings of a one-sided love. When she returns to the demon lord's castle, she finds the heroine of Regalia Of Saints, Demons, and Maidens 2, Selena Gilbert, fighting for demon incense with Auguste Zelm, the official hero...

Aileen and Claude get separated by a misunderstanding. Aileen stubbornly decides she can avoid getting executed if she just dissolves her engagement with Claude and buries herself in work. Soon, she receives a dress from Claude for the upcoming ball. He's cold when she tries to seduce him, but he chases after her once she gives up...... The confused Aileen shows up to the ball dressed as a duck, but is blown away by the words that comes out of his mouth.

Aileen falls into the sorcerer's village. While she is imprisoned, she finds out that this is all part of a coup d'état planned by the Levi family and that Lilia instigated their actions. Aileen discovers that Lilia has been secretly serving the empress dowager and she escapes the prison after winning Elephas, who has been branded as a traitor, over to her side. The two of them rush to the imperial palace to save the doomed Levi family and Claude. However, Lilia stands in their way with another holy sword in her hands.