Residents of Yuragi-sou
After Fuyuzora introduces Koyuzu to the Yuragi Inn, the two became close. Their bond can described as something similar to an older brother and younger sister. Koyuzu would often try to find various ways to help him and vice versa. They're close enough that Koyuzu is able to posses Kogarashi, which is something they can accomplish through a mutual deep trust.
Along with Kogarashi, Koyuzu is very grateful towards Yuuna by giving her a place to stay in the Yuragi Inn. As a result, Koyuzu would try to find ways to help Yuuna in any possible way. In turn, Yuuna also tries to oblige to Koyuzu's various needs including erotic ones. When Koyuzu's transformation skills improved, she was able to create a fake body for Yuuna to possess.
When Koyuzu moves to the Yuragi Inn, she finds Sagiri's boobs to be amazing. Like the rest of the Yuragi residents, Koyuzu would try to find ways to help Sagiri. An example is when Koyuzu dressed up as a Magical Girl, she helps give Sagiri smaller breasts.
The two are close friends as they share a similar interest in a Magica Girl anime. On occasion, Koyuzu would play as a magical girl with Yaya as her cat familiar.
Like with most of the Yuragi female residents, Koyuzu enjoys groping Nonko's breast. Sequentially, Nonko is among the few who doesn't mind Koyuzu's groping. Koyuzu would often use her transformation skills to assist Nonko in her manga or any other requests.
Chisaki is the girl that Koyuzu stalked because her boobs were ideal for her transformations. After hearing Koyuzu's story, Chisaki lets Koyuzu see her breasts as much as she wants, and the two became close since. Koyuzu is overjoyed whenever Chisaki comes to visit her and whenever she stays at Chisaki's house. Whenever there is an opportunity, Koyuzu tends to grope Chisaki's boobs. Koyuzu is so fond of Chisaki and her boobs that she created various things referencing her like a chocolate mold or a transformation technique. Their relationship developed to the point they consider each other as family. This allows Koyuzu to easily possess Chisaki because of their close bond.
Like Chisaki, Koyuzu is enamored by Hiyori's large breasts. Hiyori is also very enamored by Koyuzu's cute appearance and loves touching her ears and tail. Koyuzu would often embrace Hiyori's boobs the same way she does with Chisaki, much to Hiyori's pleasure.
The two became acquainted after Koyuzu accidentally undos Miria's human disguise in her attempts to infiltrate the Yuragi Inn. Koyuzu immediately identifies Miria as a bakekitsune much to Miria's displeasure who preferred to be called a Youko. The two then got into a little competition by doing various transformation techniques at each other. This eventually developed a rivalry between the two as they share similarities in both transformation techniques, age, and levels of maturity.