The Serpent Demon has layered black hair, similar to Obanai Iguro and the other members of his clan, and long, slanted pupils similar to a snake with a mouth stretching from ear to ear. Her upper torso has three violet lines alongside the oblique of her chest and has a golden lower body of a snake.
Not much is known about the Serpent Demon's personality except that she had taken a sinister liking towards Obanai, his heterochromia caused him to seem "rare" in her eyes. The Serpent Demon had a disturbing preference towards children (infants) and requested them regularly as her favorite meal.
The Serpent Demon was arguably smarter than the average Demon as she could develop a contractual relationship with humans, which in turn gave her a safe feeding ground to eat in, a steady food supply of her favorite type of "meal" and a place to hide from both sunlight and Demon Slayer for hundreds of years completely undetected.
Not much is known about the Serpent Demon's past, other than that she eventually met the Iguro Clan at an unspecified point in the past and struck a bargain with them. The clan would feed her with their offspring in exchange for riches she would acquire from her attacks. At some point Obanai was born, the rare case of a male in the clan in many years. Because of this and his heterochromia, the demon would save him for later, cultivating him for years before reaching a maturity that satisfied her.
As Obanai grew, he was brought before the Serpent Demon who had his face cut across the mouth so it resembled her own, with the blood he coughed being collected and served to her. Knowing he was in danger, Obanai secretly dug a means to escape, but as he was about to flee the Serpent Demon caught him. Fortunately, the Flame Hashira arrived and killed the Serpent Demon, saving his life.
A cousin of Obanai arrived and berated him for trying to escape as it resulted in the death of 50 members of the family and claimed he should have died instead, which he found to be irrational. Nonetheless, her words stuck deeply on him, and realizing the threat that demons posed to the world, Obanai became a Demon Slayer after the incident.
Obanai reminisces about the Serpent Demon when he sees Mitsuri Kanroji is too injured to fight Muzan Kibutsuji. Although he desires to confess his love to her he believes his blood is impure as a result of his family's misdeeds and instead would rather die and meet her in another life where he can love her freely.
Not much is known about the Serpent Demon's abilities, although given that a Hashira was dispatched to slay her, it can be assumed she was a reasonably powerful demon. However, given the past events didn't present the Hashira showing any visible struggle, it is likely that her strength was beneath that of the Twelve Kizuki.
- Kimetsu no Yaiba Manga: Chapter 188 (Page 14).