Aoi Kanzaki/Synopsis

History
Aoi's family was killed by demons. As a result, she was taken in by the Butterfly Mansion, where she then took up residence.

Aoi managed to survive the Final Selection, though she claims that it was through pure luck. She was too afraid to go in to battle despite passing the Final Selection, so she decided to work at the Butterfly Mansion instead. She seems to undervalue her own work, claiming that she isn't doing much compared to full fledged hunters.

Although she cannot fight, she is well versed in medical knowledge and was presumably taught by Shinobu. She helps with the upkeep of the Mansion and cares for injured Demon Slayers.

Rehabilitation Training Arc
Aoi first meets Tanjiro when she discovers him and the Kakushi talking to Kanao Tsuyuri in the garden. She brings them inside to the recovery beds, displaying clear frustration at the injured Zenitsu's constant wailing. She provides care for the trio throughout their stay.

During the training portion of their recovery, she explains the general ways they will be trained, including massage, reflex training, and tag games. Aoi and Kanao both play the reflex and tag games. Aoi is quickly beaten by Zenitsu, who refuses to splash the medicinal water on her. She is still displeased with him, as she had heard his rant about how wonderful it would be to train with girls, since he gets to touch them. He also beats her in tag, but she punches him when he gets too touchy.

Aoi is also beaten by Inosuke in the games, and later by Tanjiro. She becomes annoyed when Inosuke and Zenitsu stop coming to training.

When the trio depart, she tells Tanjiro that she enjoyed sharing her time with him, and wishes him good luck. Tanjiro responds by thanking her for taking care of him, but she deflects this by saying it wasn't much since she can't fight like others. Tanjiro tells her that being unable to fight has nothing to do with it and that since she helped him, he'll carry her feelings into battle as well. Aoi seems surprised by this response, but not unhappy with it.

Mugen Train Arc
Aoi once again helps care for the injuries of the trio following their fight on the train.

Entertainment District Arc
Aoi and Naho Takada are both picked up by Tengen Uzui, who wants them to help infiltrate the Red Light District. Kanao Tsuyuri attempts to stop the Hashira from taking the two girls away, but having trouble with her own lack of resolve, she does not say anything to stop him. Tanjiro interrupts the commotion and asks Tengen to release the girls, to which Tengen responds that he needed a female Demon Slayer for his mission and had planned on taking Aoi and Naho. When one of the younger girls explains that Naho was not a Demon Slayer, Tengen drops her and instead plans on only bringing Aoi. Aoi appears distressed and upset when Tengen commented that even if she wasn't useful, she was still a member of the corps. Tanjiro, Inosuke, and Zenitsu eventually volunteer to go in Aoi's place, releasing her.