The Butterfly Mansion in Demon Slayer serves as a hospital and rehabilitation facility for injured demon slayers. Originally belonging to the Kocho family, the mansion and its staff treat the many ailments of the Demon Slayer Corps and ensure that they are in fighting shape again to get back to work.
The main caretaker at the mansion is Aoi Kanzaki, a bold and hot-tempered young lady who takes her job very seriously. Tanjiro and the gang have a lot to thank her for, for it was her care that healed them many times over. However, did you know that Aoi is actually a trained demon slayer, herself?
Introducing Aoi Kanzaki
Aoi makes her first appearance after the Master, Kagaya Ubuyashiki, decides that Nezuko can fight alongside the Demon Slayer Corps. Subsequently, the kakushi take Tanjiro to the Butterfly Mansion to be healed. When they arrive, they are met with immediate suspicion from Aoi as to what they are doing there. She seems scary and someone who is not to be messed with; when she instructs them to follow her, Tanjiro even looks slightly scared. After they reach the infirmary, they find Zenitsu throwing another hysterical fit over taking his medicine. Aoi quickly puts a stop to this by yelling at him to be quiet and to drink up, otherwise, she will tie him up and force-feed him.
This is the perfect introduction to Aoi. She stands for no nonsense, is slightly impatient and serious. Her temperament is clearly one that is short-tempered and blunt. She knows she has a job to do and intends to do it properly, regardless of who puts up a fight. Not only does she deal with hands-on treatment, but she also ensures that the rooms are clean, cooks nutritious food for the patients and even designed her own rehabilitation regimen for Tanjiro, Zenitsu and Inosuke. Her role as a nurse is taken very seriously and with a lot of care, and her results are undeniable. She is the Corps' secret weapon, along with the rest of the staff at the Butterfly Mansion.
Aoi is Actually A Member of the Demon Slayer Corps
You might notice that Aoi wears a demon slayer uniform under her nurse's one. According to the official Second Fanbook, Aoi is trained in Water Breathing, the same as Sakonji Urokodaki, Giyu Tomioka and Tanjiro Kamado. With this training, she made it to Final Selection and survived.
So Why Doesn't Aoi Fight?
It is revealed by Insect Hashira Shinobu Kocho that all the staff in the Butterfly Mansion had families that were killed by demons, including Aoi. While she trained and survived Final Selection to fight these monsters, she claims it was by pure luck. When Tanjiro thanks her for treating him and his friends no matter how busy she was, she replies by saying she does not think she was much help at all. According to her, she was a "wimp who just got lucky". She was deeply afraid to fight in the Demon Slayer Corps after that ordeal.
In season 2 of Demon Slayer, Sound Hashira Tengen Uzui tries to take Aoi (forceably at that) on his mission to infiltrate the Entertainment District, as he needs a female demon slayer to blend in. Aoi's face falls when he says that she does not look very useful but because she is a demon slayer nonetheless, he will settle for her. Aoi evidently has some low self esteem or maybe even guilt for not fighting on the frontlines with her peers. She believes that she is a coward and hearing someone elude to that out loud must leave a sinking feeling in her. Tanjiro defends her decision, yelling at Tengen that everyone has their reasons and circumstances for making decisions, so he must not be insensitive to that.
Despite her training and passing of Final Selection, Aoi chooses to serve the Corps in a different way, by making sure that its members are in optimal fighting shape and health. Presumably, Shinobu must have taught her and the rest of the staff how to properly care for the variety of injuries inflicted on demon slayers. Aoi has done her studying and now is the main caretaker at the Butterfly Mansion. Aoi is an absolute perfectionist at her job and if it was not for her, Tanjiro and the others will not be able to continue their adventures towards defeating Muzan Kubutsuji once and for all.