Helene

Helene is a witch who's trapped in a portrait in the  Demon Lord's Castle. She is first introduced in Lesson 7-12.

Story
(spoilers ahead)

Background
Helene was a witch who's beauty attracted many men, including angels and demons. She was then seduced by Asmodeus and had an affair with him. Because of that, a war broke out which destroyed the country and she was proclaimed to be an evil woman. As a result, a sorcerer named Demetrios, who was the nephew of her former lover, sealed her inside a painting.

She spent hundreds of years trapped and blamed Asmodeus for her fate.

For reasons unknown, she ended up in the Devildom and placed inside the castle. It was presumed that she kept to herself since no one knew the portrait could speak but would suck unsuspecting patrons inside her portrait.

Main Story
During the retreat, Diavolo, the brothers, MC, the angels, and Solomon were touring the castle and learning about its history when Helene recognized Asmodeus and unleashed a scream that brought everyone's attention towards her. She then lashes out at him but Asmodeus is unremorseful for he does not feel guilty about the damage he caused to her, even openly flirting with her.

As an act of revenge, she sucked Asmodeus, Mammon, Leviathan, Beelzebub, Solomon, and MC into her painting, sending them to the Castle's labyrinth where Henry 1.0 resides.

Trivia

 * She's likely based on Helen of Troy, for her beauty was unparalleled and had a lover before entering an affair that led to disastrous consequences.
 * Despite being a portrait, she's the only female character to visually appear in the game.