Egen is a yōkai from the Underworld traveling to the human world to observe Hanae Ashiya's high school. Her goal is gathering insights for establishing an Underworld school, a mission arranged by Gyōsei, one of the Underworld's three rulers, so she can repay a debt owed to him.

She possesses reddish-pink skin, round eyes, and long white hair parted at the back. Egen typically wears a haori, a large scarf covering the lower half of her face, and a head accessory. She consistently carries a notepad to document her observations.

Egen is earnest, honest, and deeply dedicated to her educational goals, though she struggles with deception. Observing the human school, she expresses surprise at the students' literacy and the teachers' expertise, noting educational gaps in the Underworld.

Unbeknownst to others initially, Gyōsei also assigned her a secondary objective: to discreetly monitor Ashiya and Haruitsuki Abeno. This dual purpose strains her interactions, especially after Abeno discovers her spying role. He initially avoids involving Ashiya in guiding her, creating tension until her full mission is revealed. Her espionage contributes to Gyōsei's decision to ban Ashiya from the Underworld due to distrust of humans.

A prior debt to Gyōsei motivated Egen's acceptance of the task. Her development centers on reconciling her genuine passion for education with the complexities of her obligations to the Underworld's rulers.