Affably Evil: Ai's teammates have a fairly even-minded opinion of human kind, and happily support their co-workers, students, and friends in their many, many jobs. It almost makes you forget what their real job is.
Clark Kenting: Justified, since they are able to prevent mortals from recognizing them. It becomes a plot point in the third season.
Hidden Heart of Gold: Despite their efficiency and cruelty in sending people to hell, they can be quite kind-hearted and sympathetic to the turmoil of others. Not often, this may include gentle nudges to use Hell Correspondence if they believe it would save the person from further suffering.
Not So Stoic: They are not allowed to have their emotions interfere with their job; but on certain occasions this will happen to one of them.
Punch Clock Villains: Very much so. Ai ferries people to hell because that's where her parents will go if she doesn't. Her assistants stay with her out of loyalty.
The titular Hell Girl. It is her duty to offer a devil's deal of vengeance to any who access the Hell Correspondence. If the "customer" accepts, she ferries the target of that person's vengeance to Hell. As part of the deal she will also ferry the "customer" to Hell after he or she dies.
Heroic Sacrifice: When she refuses to transport the soul of Takuma Kurebayashi, whose life situation somewhat mirrored her own. As a punishment she becomes mortal and later dies while trying to defend the boy from violent townspeople.
Hime Cut: It's meant to give her the air of a traditional miko... but in a rather dark and unsettling sense.
Human Sacrifice: Her backstory; Ai was meant to be this as a little girl, but with Sentaro's help she was able to stay alive even when abandoned in the mountains - until the villagers found out and took it upon themselves to bury her alive along with her parents. In revenge, she rises from her grave and burns down her village, and is made into the Hell Girl as punishment.
Meaningful Name: Enma is the Buddhist god who rules the underworld. Ai is that word for love, probably. It's deliberately always written in kana, so we can't be sure which meaning of ai her parents had in mind. There are hints that the "indigo" meaning is significant: the second season finale is called "Aizome" ("dyed in indigo"), as is the end theme, which Mamiko Noto herself sings.
One of Ai's assistants. He appears as a young man.
And I Must Scream: As a tsukumogami, he was a sword that became sentient after one hundred years. Because he was still an inanimate object, he was Forced to Watch as people used him as a weapon of both war and murder. He had been left behind after a battle and was stuck there until Ai found him.
Dark and Troubled Past: She was originally a young woman named Tsuyu, who was sold into prostitution by her lover to pay off his debts. When she tried to plot an escape for her fellow prostitute Kiyo, she betrayed her and had her killed. When her corpse was thrown into the river filled with bones, the spirits fused with her body, becoming Hone Onna.
Dem Bones: Being based off the mythical youkai of the same name, she can make any part of her skeleton visible.
Dull Eyes of Unhappiness: Subversion; while they reflect her unhappy origins as a youkai, she is the more expressive in the group.
Sins of Our Fathers: Near the end of the first season, Ai tried to kill Hajime and Tsugumi because they are descendants of Sentaro. When that didn't work, she tried to bully Tsugumi into banishing Hajime to hell. It is also possible that Ai simply mistook Hajime for Sentaro.
The Cameo: She is seen as a slightly older schoolgirl near the end of the second season when she tells someone about the book her father Hajime wrote before he disappeared.
Disappeared Dad: From the second season onward. It's revealed that Hajime and Tsugumi gradually drifted apart and lost touch; and in Season 3 she confides to Yuzuki that she no longer has any idea of where he is. She does recall that he simply became "tired" of trying to stop people from condemning themselves to hell in the name of revenge.
First Name Basis: With her own father. Some of their interactions actually make them seem more like siblings.
Forced to Watch: She is somehow able to see through Ai Enma's eyes as the Hell Girl meets with her clients, and she uses this ability to help Hajime track the clients down. She keeps this ability as an adult.
Screw Destiny: As an adult, she believed her visions meant that she was fated to become the next Hell Girl, and she managed to escape it.
Sins of Our Fathers: Ai tried to kill Hajime and Tsugumi because they are descendants of Sentaro. She then tried to bully Tsugumi into banishing Hajime to hell. It is also possible that Ai mistook Hajime for Sentaro.
A boy who lives in the town of Lovely Hills. He is blamed for murdering his mother and assaulting his father due to being in the wrong place at the wrong time in both cases. When people begin disappearing around town, the townsfolk blame him. As the disappearances continue, people begin to fear that he has supernatural powers and begin calling him the "Devil's Child."
The younger sister of the detective investigating Takuma. She befriends him after finding out that he isn't responsible for the disappearances around town.
Despair Event Horizon: After her brother gets banished, she tried to banish Takuma and then drown herself so she can join them in hell.
Driven to Suicide: After attempting to banish Takuma, she wades out into a lake so she can freeze to death and be with Takuma in hell. She survives, but is hospitalized.
Freak Out: When Kikuri brings her to Ai's hut, she looks behind the sliding door and gets a good look at Ai's "grandmother." She runs away in terror. Then Kikuri drags her behind the sliding door again.
Heroic BSOD: When her brother is banished, she accesses the Hell Correspondence. She thinks that with Takuma gone, things in her hometown will calm down.