Why Aoi is not a Demon Slayer, explained
Aoi Kanzaki is a curious character in Demon Slayer, one of the few who passed the Final Selection test period for would-be slayers, and one who utilizes the ‘water-breathing’ technique. She has ties to the Insect Hashira, Shinobu Kocho, and generally acts as a support for the protagonists after their brutal fights.
Despite all this, Aoi isn’t actually one of the eponymous slayers. Fans might wonder why this is, since she does seem like she would be a full-fledged fighter under other circumstances. But Aoi’s position, or lack thereof, is her personal choice. She says as much in the anime herself, that she simply can’t bear going into battle.
Aoi’s reluctance to become a Demon Slayer
Aoi is first introduced following the Final Selection test on Mount Fujikasane, when Tanjiro and the fellow survivors of the test are taken to the Butterfly Mansion. She ends up informing the trio of Tanjiro, Inosuke, and Zenitsu about their training within the Mansion as they become full-fledged Demon Slayers.
Though her opinions are only really positive on Tanjiro - Zenitsu being weird about girls and Inosuke’s laziness annoy her. Fittingly, Tanjiro is the only one of the three she opens up to. It’s here that she confesses she cannot fight, and feels like she can’t be as helpful as the others. This is the crux of her reluctance to be a Demon Slayer - she doesn’t believe in her own strength.
Regardless, Tanjiro assures her that she was helpful to him, and he’ll carry her feelings into battle in exchange, much to her surprise. It’s a nice establishing character moment between the two that elaborates on Aoi’s feelings on her role. She continues to be useful as when Tanjiro and friends return from the Mugen Train incident, she helps nurse the returning slayers back to health.
Aoi feels inadequate compared to others in the Corps
Aoi later gets roped in by the Sound Hashira, Tengen Uzui, to help infiltrate an entertainment district where his agents had gone missing. Initially he was going to use her as a spy, as she was technically a Demon Slayer, but eventually she got bailed out by Tanjiro and friends. His disregard for her indicates that some do pay little heed to her importance, which gives more weight to that earlier conversation.
Some may call Aoi a coward for shying away from combat, but in reality, it seems like she just has a different way of helping. It was through her help that the main trio of demon slayers first trained within the Butterfly House, and she also helped them recover as they returned from more and more dangerous missions.
To be frank, no one can really fault her for not wanting to fight either - she believed her survival of the Final Selection to be a mere fluke, and that presumably soured her off the idea of fighting in the constant war against Muzan’s army of the night. Most readers probably wouldn’t handle the world of Demon Slayer very well, vengeance-driven or not.
Aoi may not technically be a slayer, but she does serve an essential part in the Demon Slayer Corps. Her help around their headquarters provides a way for the hard-working slayers to recover and recuperate following their violent night missions, even if she herself doesn’t have the strength to face the demons herself.