Also known as the Hell Girl, Ai is the preternaturally expressionless young girl who comes to deliver vengeance on behalf of those who submit their grievances to HellCorrespondence.com. Though not much is known about this mysterious young maiden, her fate is one of eternally ferrying the damned to Hell, as atonement for some dark deed done... Something tragic surely lies in her past.

Living apart in a world of perpetual twilight, a faceless grandmother, a silent spider and a computer are Ai's only constant companions. When the time comes for vengeance to be delivered, Ai is assisted by three beings under her service, perhaps as mysterious as she. And from time to time, the occasional task at hand or victim will appear to pique her interest...

One of Ai's assistants, in human form Wanyuudou appears as a man of considerable age, dressed in relatively out-of-date attire. The red scarf he wears is suggestive of the red string around his neck when Wanyuudou takes on the form of the black straw doll, given to the recipient of Hell Girl's services. His voice rings out eerily with his release, "Your grievance shall be avenged." Wanyuudou also serves as Ai's coach between the worlds, his face in the fiery wheels and Hell Girl's mark upon its walls...

Another of Ai's assistants, Ichimokuren most often appears as a handsome young man, even displaying a bit of vanity from time to time. He can also take on the form of a single, sizable eye, able to pierce deep within the confines of any structure to spy upon the deeds done by the soon-to-be damned. His interest and occasional puzzlement of human interactions hint at something non-human in his past...

The last of Ai's assistants, Hone Onna is a devastatingly beautiful woman dressed in the traditional attire of a geisha. As with Ichimokuren, she is prone to bouts of vanity, but only when her age is called into question. Hone Onna will occasionally dress in casual clothes to enter human society in order to investigate the soon-to-be damned, often with Ichimokuren in tow, and even sometimes Ai herself...

Grandmother - Never actually seen, Ai's grandmother is always present in the world of twilight in which the two live. No more than a silhouette on the rice paper screens, she is ever spinning, spinning thread at the wheel. She will occasionally remind Ai of another client to be served, letting the young girl know that her nagajuban has been prepared (a white garment worn beneath a kimono).

A tabloid journalist, Hajime seems to make his living by blackmailing celebrities who wish to keep their scandals from going to print. That is, until he comes across rumors of the website HellCorrespondence.com. Having seen with his own eyes the truth behind the hype, Hajime becomes obsessed with finding out all that he can, often to the detriment of his paying job. With the help of his daughter, Tsugumi, he often works to undermine Hell Girl's own work by convincing her clients not to seek revenge. There must be some connection between the two that goes beyond Tsugumi...