Shido Itsuka[]
Shido is Kotori's adopted older brother. Her deep love and care for her brother have been shown many times. However, she treats her brother harshly when in commander mode but clings to him heavily when in little sister mode. She will normally call Shido “Onii-chan” while wearing white ribbons and simply use his given name while wearing black ribbons. Nevertheless, her latter persona will occasionally slip back to “Onii-chan” in moments of emotional vulnerability.
Kotori developed her commander mode persona when Shido gave her black ribbons as a birthday present and made her promise to be strong when wearing them since she proved to be quite a crybaby without them. However, when she is wearing her white ribbons or no ribbons at all, she returns to her persona as his younger sister, which proves to be Kotori's true self. A lot of Shido's peers actually find it strange how close they are. According to the Fraxinus' machines, her affection points for Shido neither rose nor fell during their date to seal her Spirit powers: because said points were already maxed out the entire time.
While having Shido seal Reiryoku, she was secretly aware that she may eventually have to kill him if he ever lost control of his powers. Despite being deeply conflicted, she seemed to accept the duty with resolve. When Shido goes berserk, Kotori tearfully prepares to kill him with <Dáinsleif>. But luckily, she is stopped by the other Spirits. Kotori feels guilty for being the first Spirit Shido ever sealed, starting him on the path of a tool seen as disposable by many of Ratatoskr's higher-ups.
Despite her usually blunt and harsh attitude, Kotori's feelings for Shido run deep. They have been shown numerous times to go beyond the boundary of sibling love, to the point of being romantic. During <Beast>'s assault on Tengu City, she loses her temper toward him for wanting to confront the savage Spirit without his powers, ultimately confessing her love for him and her fear of putting him back into danger. After Shido assures her that his love for her as a sister means no less than romantic love, she decides to combine her ribbon colors, and consequently, her two personalities, to approach Shido as both a loving sister and woman.
Overall, Kotori can be considered a tsundere towards her older brother, often teasing him and even acting a little sadistic towards him while in commander mode. As a running gag, when either "little sister" or "commander" mode hits Shido, she makes puns on whatever odd noise comes out of his mouth as a result.
Reine Murasame[]
Reine is the most reliable person that Kotori depends on when she needs to ask for advice. It has been stated several times that she is Kotori's best 'friend'. She was incredibly distraught when Reine revealed herself to be <Phantom> and an alter ego for Mio Takamiya, questioning the latter before being killed in the Game Over Timeline if they had ever truly been friends at all. She was reluctant to accept Shido's explanation about Reine's identity in the following timeline, but she still set up a date to seal Reine and keep her on their side. Kotori was mournful when Mio (and Reine by extension) sacrificed herself to save everyone from Westcott. Even one year after her death, she would still keep Mio's teddy bear at Reine's seat in the <Fraxinus> as a memento.
Kyouhei Kannazuki[]
Although Kyouhei is the Vice-Commander of <Fraxinus> under Kotori's command, she treats him poorly due to his perverse antics and small obsession with her, leading to her ignoring him most of the time. However, she is also aware of Kannazuki’s talent with the Realizer and trusts him to command <Fraxinus> when she is not present.
Fraxinus's Crew[]
Kotori treats all of her subordinates fairly as their commander. It's noted by her that she sees most of her crew as a family. She has memorized the names and ranks of every single subordinate, even the ones she has barely had any direct contact with whatsoever.
Mana Takamiya[]
Kotori and Mana are both rivals at being little sisters to Shido. However, after Mana defects from DEM and stops a hypnotized Kotori from killing Shido, their relationship has greatly improved to the point that Kotori is begging Mana to receive treatment for her body. As such, Kotori has now come to see Mana as a part of her family as well.
Yoshino Himekawa[]
Yoshino is grateful for the assistance that Kotori provides for the Spirits, with that gratitude a key reason why she was willing to use her powers to save Kotori from Origami. On Kotori's side, she remarks Yoshino is a "scary child" upon realizing the seduction tactic she employed for Shido. Despite this, Kotori is aware of Yoshino's strengths. While surprised that Yoshino was the winner of the Spirit battle royal, she had no doubts over Yoshino's capabilities for victory.
Kurumi Tokisaki[]
Kotori is wary of Kurumi's status as the Worst Spirit and her willingness to harm civilians outside of Spacequakes. When witnessing the threat that Kurumi is capable of causing in terms of causalities, she even uses her last resort of taking back her powers from Shido to personally confront Kurumi. Nevertheless, Kotori still is willing to help Kurumi due to Ratatoskr's overall goal being to save every Spirit, with her out-of-character attempt at Kurumi's life only due to the destructive impulses from using her powers taking over. For her part, Kurumi remains cautious of Kotori after their fight, preferring to avoid another fight with the Spirit of Fire if possible.
Origami Tobiichi[]
Origami mistook Kotori for the Spirit that killed her parents and tried to kill her. Kotori displayed similar killing intent as a result of the destructive urges from using her powers, but this quickly changed to horror when accused of murdering her parents. Afterward, Kotori remained wary of Origami and tried to avoid contact with her. Despite this, she still assisted her against Ellen due to their shared desire to protect Shido.
Their relationship has softened in the new timeline where Origami no longer hates Spirits, with both girls now addressing each other by first name instead of full name. Nevertheless, she is still appalled by Origami's perverted behavior toward her older brother.
Natsumi Kyouno[]
Out of all the Spirits Shido has sealed, Natsumi appears to be the one Kotori is closest with. Unfortunately, much of their interactions stem from Kotori finding herself as the regular target of Natsumi's pranks.
Miku Izayoi[]
Before Miku was sealed, Kotori described her as being "twisted" due to how she was full of contempt when looking down upon other people. Later, she was among those brainwashed by her <Solo>, although this was quickly undone due to Mana knocking her unconscious.
After Miku is sealed, Kotori is one of the few Spirits who get annoyed rather than scared of Miku's yuri tendencies. Also, Kotori is not above using physical force to make Miku back off whenever she cuddles up to her.
Mukuro Hoshimiya[]
Kotori appears to be envious of Mukuro due to her having a very busty figure even though they are close to the same height. This jealousy was specifically shown when Mukuro was lent Kotori's clothes and complained about the chest portion being too tight.
Elliot Baldwin Woodman[]
Kotori holds a great deal of respect toward Ratatoskr's founder due to his devotion to protecting Spirits and being one of the few higher-ups who cares about Shido as a person rather than a means to that goal. During meetings with him, Kotori will properly wear her military uniform, something which she does to indicate Woodman as a superior she holds in high esteem. He is the only member of Ratatoskr's Round Table whom she holds in high regard, representing him with an ordinary toy squirrel at her meetings instead of the intentionally hideous stuffed animals for the other members. Elliot entrusted her with the key to activating <Dáinsleif> because he suspected the other higher-ups would be too quick to fire the weapon if Shido lost control of his sealed Reiryoku.
MARIA[]
As the Fraxinus AI, MARIA was built to assist Kotori in her role as commander of Fraxinus. While the AI provides a supporting role, she is willing to criticize Kotori's decisions if necessary. For her part, Kotori does treat MARIA like family since MARIA's systems were created by her parents. For instance, similar to any relative, Kotori found herself exasperated when she discovered MARIA obtained a secret part-time job to finance her own needs.