Ai Enma originated in early 1600s Mutsumi Village, where she faced relentless bullying as a child due to superstitious fears surrounding her alleged supernatural abilities. Her cousin, Sentarou Shibata, consistently defended her. At age seven, village elders selected Ai for ritual sacrifice during the "Seven Sending" ceremony to appease a mountain god. Her parents resisted and arranged for Sentarou to hide her in the woods, where their bond deepened into unspoken romantic feelings. When crop failures plagued the village, rumors guided the villagers to Ai's hiding place. They captured her and buried her alive alongside her parents, compelling Sentarou to participate in covering her grave. This profound betrayal ignited Ai's intense hatred.

After clawing free from her grave, Ai exacted revenge by burning Mutsumi Village to ashes, sparing only Sentarou, who had fled. Witnessing the inferno, Sentarou laughed ambiguously—potentially from relief at her survival or satisfaction at her vengeance. This act drew the attention of Hell's master. He punished Ai by holding her parents' souls hostage, offering a pact: in exchange for their souls escaping damnation, Ai would become an eternal messenger of vengeance. Bound by this agreement, she was forced to suppress her emotions and observe human suffering without interference, accepting the role of the first Hell Girl.

For over four centuries, Ai ferried souls to Hell while existing in a twilight dimension, appearing perpetually as a thirteen-year-old girl with pale skin, waist-length black hair in a hime-cut, and red eyes that replaced her original brown irises after her burial. In her daily existence, she typically wore a black sailor uniform. When fulfilling her duties, she donned a distinctive kimono featuring seasonal designs: large floral patterns in the first season, sakura petals in the second, animated fans and toy balloons in the third, and black fabric adorned with flowers and butterflies paired with a golden obi in the fourth. Her methods of contact evolved from Ema tablets to newspaper ads and finally a modern website called Hell Correspondence, accessible only at midnight by those consumed by intense hatred.

Ai initially displayed a cold, detached personality, speaking minimally and showing no emotional response to human suffering—a direct consequence of the emotional suppression enforced by Hell's master. Over time, subtle shifts occurred: she occasionally mocked clients before damning them, displayed rare concern for innocent victims, and once reassured a mother that her companion Ren would protect the woman's daughter. These moments hinted at underlying emotions beneath her largely stoic demeanor. The suppression proved imperfect; when forced to condemn an innocent person, she internally screamed in anguish, and tears occasionally welled in her eyes. Her buried hatred surged most violently when confronting descendants of Sentarou Shibata, leading her to burn a temple he built and attempt to coerce his relative Tsugumi into seeking vengeance.

During the events of the third season, Ai vanished temporarily before returning unconventionally: she possessed the body of schoolgirl Yuzuki Mikage after kissing her during a trance-like state in a bathtub. Using Yuzuki's form, Ai resumed her duties, ferrying a teacher to Hell via the Hell Correspondence website. This possession entangled Yuzuki in Ai's work, forcing her to witness and participate in the cycle of vengeance despite her distress over being chosen. Ai's companions—Wanyuudou, Ichimokuren, and Hone Onna—resumed assisting her, joined by a new yokai, Yamawaro. Throughout this period, Ai continued operating from the twilight realm, using her connection to Yuzuki as a conduit.

The unresolved tension from Ai's human life persisted through centuries. She encountered Tsugumi Shibata, establishing a telepathic link allowing Tsugumi to see through Ai's eyes and identify vengeance targets. This connection culminated in Ai confronting Sentarou's modern-day descendants, Hajime and Tsugumi Shibata, where she attempted to force them into the cycle of hatred. Tsugumi's choice of forgiveness thwarted Ai's efforts. Later, confronting the entity Gilles de L'Enfer, Ai's suppressed memories fully resurfaced, intensifying her turmoil. After escaping a forced journey to Hell on a boat piloted by Hell's master, she ultimately resumed her eternal role, bound to ferry souls while wrestling with the fragments of her humanity beneath her impassive exterior.