Twin sister of Eustace. A good student in terms of her grades, and is described by those around her as incredibly serious. She is passionate about her position as a manager for NRC’s basketball club.

Alethea has a slender stature with long, shoulder-length, semi-wavy, jet black hair that reaches down just below her waist that she ties into low pigtails, and bangs that reach the top of her eyes. She has a freckled face and the sides of her face are framed by two long bangs.

Her uniform is similar to the rest of the Diasomnia students. Alethea wears her hair in an updo. She also wears dark green eyeliner while in her dorm uniform. Her uniform consists of a black all-weather coat with neon green and silver accents that reaches above the knee in length, black shorts inside the coat, black combat suspenders and belts, a small garrison cap, black gloves, navy stockings, and heeled knee-high boots.

Alethea’s unique magic is called Rose Thorn. She can cast wall of thorn that serves as shield. From others' perspectives, her unique magic is similar to Eustace but her unique magic emits the scent of roses and she can cast her thorns around people to trap them, which Eustace cannot.