Butterfly Girls

Butterfly Girls is the friendship between Kanae Kocho, Shinobu Kocho, Kanao Tsuyuri and Aoi Kanzaki from the Demon Slayer fandom.

Canon
While Kanae died years prior to the current story. She is confirmed to have been a former resident of the Butterfly Mansion estate. She was a very kind and caring woman. As stated by Shinobu. The other residents at the Butterfly Mansion viewed her as a motherly figure. After Kanae’s death, Shinobu's anger for Demons grew. But she thrived to keep the smile her sister had always loved and she wanted to become that kind person her sister once was.

4 years prior to the current story. Shinobu and Kanae had adopted Kanao. Who was abused by her parents and then later sold into slavery, where she had first met the two butterfly siblings. But Kanao lacked emotions and never really did anything on her own and would only do what she was told.

This worries Shinobu but Kanae resolves the problem by giving Kanao a coin and tells her to flip the coin to decide what to do when she’s on her own.

Aoi was also taken while Kanae was still alive. As she is slightly responsible for why Kanao has her name. When the name selection for Kanao took place, Shinobu and Kanae placed a bunch of random different cards with potential names for Kanao. Once she saw that Kanao was going to pick the one that said "Kamasu" Aoi swapped it out for the card that had Kanao instead. So while the two haven’t had any onscreen interactions. It is confirmed that Aoi did live there when Kanae was still alive.

Shinobu treats both Kanao and Aoi very kindly at the Butterfly Mansion. Ever since Aoi gave up on becoming a Demon Slayer. Shinobu has shown no indications of being upset with her for making the decision and chose to use her medical knowledge to help Aoi become skilled at treating those who were wounded after missions. Shinobu also has a strong sympathy for Aoi and the others such as Kiyo Terauchi, Naho Takada, and Sumi Nakahara. Because their families were killed by Demons as well, believing they would be living very happily with them if they weren’t slain by Demons.

Kanao was raised and trained by Shinobu after Kanae’s death. Shinobu’s training was able to allow Kanao to pass Final Selection and become a successful Demon Slayer. Shinobu’s training also was able to allow Kanao to rise up the ranks and even make it to Tsuchinoto. Kanao became so strong. That she was able to outclass Zenitsu Agatsuma, Inosuke Hashibira, and even Tanjiro Kamado, couldn’t match her strength at the time.

Shinobu also shows a protective side of those who live at the Butterly Mansion. As she had a dispute with the Sound Pillar, after she heard that he attempted to kidnap Aoi and Naho for his mission. Overall the four haven’t had any scenes together. But their individual respect for one another is clearly acknowledgeable.

Fanon
The ship between these four is fairly recognized by the fandom. There's a decent amount of artwork about that consists of the four all together. These four are not to be shipped in a romantic sense however, considering Aoi and Kanao were both adopted and taken in by them so shipping them romantically would be greatly frowned upon by several Demon Slayer fans. It doesn't have any works on AO3.