In the past, Kotori resented Kotarou for his violent ways and told him that he was living a life without meaning. However, little by little both of them start seeing how similar they really are, and become something akin to friends. Kotori saves Kotarou when he is wounded by Kagari by infusing him with a piece of Kagari's ribbons and by making him into a half-familiar. This makes their relationship become awkward as they grow older, since Kotarou develops feelings for Kotori, but she can't quite tell if those feelings are real or if Kotarou is simply being influenced by her status as his summoner. In their high school days, both of them have a friendly relationship, but Kotori still keeps him at a distance.
As a Druid, Kotori has the duty to protect the Key no matter what. This is why Kotori spends most of her free time in the forest, making familiars and barriers to keep her safe. They don't seem to have a very deep relationship, since Kagari can't really speak or communicate all that well.
Before they became familiars, Kotori had a pretty good relationship with her parents, even if they could be strict at times. When Kotori announces that she doesn't want to go to the Martel meetings anymore, they agree to this. However, they do make her take care of a poor abused dog in return.
Despite this, Kotori seems to love them enough to bring them back as familiars when they die in a crash accident, and at first she harbors the hope for her to be able to bring them completely back to life eventually.
As Kotori refused to go to any other Martel meetings, her parents decided to have her take care of a dog that had been previously abused by its owner, Pero. However, due to its abuse, it was unable to understand her kindness, and so it rejected her completely. After its death, Kotori uses her Druid powers to bring him back as a familiar.
In its familiar form, Pero resembles a tiny mammoth, which is why Kotori changes its name to Chibimoth. Chibimoth is a lot more cheerful and effusive than Pero ever was.
Kotori gets along with Chihaya very well. Since they are classmates and both of them are carefree, they are regularly seen together. At some point, Kotori even considered marrying Chihaya to get to live in her residence. However, she decides against it since her name would then become "Kotori Ohtori".
The two of them get along, although never to the point of being close. In Chihaya's Route, Akane can be heard threatening Kotori in order to obtain the Key.
The two of them get along, although never to the point of being close. Kotori mediates between Lucia and Kotarou, and also between Lucia and Chihaya whenever she's able to.
The two of them get along, although never to the point of being close. In Shizuru's route, Kotori approves of Shizuru and Kotarou's relationship, even if she seems a little bit weirded out by it. Later on, she bonds both Shizuru and Kotarou together as summoner and familiar, so that Kotarou can survive.
Kotori, Kotarou and Yoshino are the main trio of friends we get to see in the first days of the story. They often talk together, although it's mostly prompted by Kotarou. All of them are good friends due to the three of them being loners. Yoshino secretly has feelings for Kotori, though he reluctantly admits it.
At the end of the Terra Route, Touka finds and raises Kotori along with the rest of the heroines.