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 proclaimed by the people 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, until she heard Asmodeus’ voice while on the tour. There she unleashed her anger toward him, but Asmodeus was friendly toward her and did not feel guilty for the damage he caused her.

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

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.