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.

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 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.

Story
During the retreat, Diavolo, the brothers, MC, the angels, and Solomon are touring the castle and learning the history. When Helen recognizes Asmodeus, she unleashes a scream bringing everyone’s attention toward her. She verbally lashes out, but Asmodeus is unremorseful for he did not feel guilty for the damage he caused her, and openly flirts with her.

As an act of revenge, she sucks 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.