Ai Enma serves as the primary Hell Girl, managing the Hell Correspondence system that allows individuals to request vengeance by sending tormentors to Hell. Her origins lie in Mutsumi Village during the early 1600s, where she endured severe bullying and ostracization fueled by rumors of supernatural abilities. At age seven, villagers selected her as a human sacrifice for the "Seven Sending" ritual. Her parents and cousin, Sentarou Shibata, helped her survive by hiding her in the mountains. Villagers discovered her six years later and buried her alive alongside her parents, forcing Sentarou to participate. This betrayal ignited profound hatred, enabling Ai to emerge from her grave and burn the entire village to ash, sparing only Sentarou.

Following this act, the Master of Hell (manifesting as a spider) intervened. He offered a pact: Ai would become an immortal agent of vengeance, ferrying souls to Hell, in exchange for sparing her parents from eternal damnation. As part of this agreement, she suppressed her emotions and personal grudges, becoming a detached observer. Her brown eyes turned crimson, symbolizing her transformation, and she gained agelessness, retaining the appearance of a thirteen-year-old for over four centuries.

In her duties, Ai appears to clients at midnight through evolving mediums—initially Ema tablets, later newspaper ads, and eventually the "Hell Correspondence" website. She presents a straw doll with a red string, explaining that pulling it sends the target to Hell but also condemns the client to Hell upon their natural death. When enacting vengeance, she dons a distinct black kimono—its design changing each season—and recites a ritualistic chant before transporting souls via a ghostly boat. Her methods involve tormenting targets with personalized illusions reflecting their sins.

Outwardly stoic and emotionless due to the Master of Hell's mandate locking away her feelings, Ai rarely speaks unnecessarily. Subtle emotional shifts occur across the series. Early seasons reveal repressed humanity, like shedding tears when forced to send an innocent to Hell or showing concern for a self-sacrificing mother. Her hatred resurfaces when encountering Sentarou's descendants, leading her to manipulate circumstances towards their damnation, though this subsides if they demonstrate remorse.

Her character evolves significantly in later seasons. She defies the Master of Hell by refusing to send an innocent boy, Takuma Kurebayashi, to Hell. This defiance costs her immortality and powers, rendering her mortal. She dies shortly after saving Takuma from a mob, her body dissolving into cherry blossoms. She later returns non-corporeally, possessing a schoolgirl named Yuzuki Mikage. Through Yuzuki, she orchestrates events to pass the Hell Girl role to her, briefly serving as an advisor. When Yuzuki struggles under the role's burden, Ai reclaims the position to spare her successor from eternal damnation.

Ai commands three primary supernatural companions bound to her: Wanyuudou, a wheel-spirit tsukumogami acting as her carriage; Ichimokuren, a sentient katana spirit using his detached eye for surveillance and combat; and Hone Onna, a skeleton disguised as a beautiful woman specializing in infiltration and seduction. A fourth companion, Yamawaro, joins in the third season as a protective demon in boy form. Their dynamics reveal Ai's capacity for loyalty; though rarely expressive, she treats them with respect and interdependence, viewing them as familial figures. Another entity, Kikuri—a childish yet manipulative force from Hell—alternately aids and interferes, symbolizing a darker aspect of vengeance.

In non-anime adaptations, like the 2019 live-action film, Ai's portrayal diverges slightly. She is portrayed as a young adult, and her methods involve more visceral, fear-inducing confrontations. The film also explores her interactions with clients like Yamada Kaede, emphasizing her role as an eternal arbiter of vengeance across different eras.