Ogre: Ogre was her captain as well as her mentor, who she held in high regards and was often sparring with. The death of him left a major mental scar as it ultimately was her reason to hunt down Night Raid. It was also Ogre who introduced her to Stylish and his experiments in order to become stronger. Due to her blind hero worship of him, Seryu was noted to be completely oblivious and ignores all of Ogre's bloodthirsty abuse of his power despite the obvious evidences of them.
Imperial Guards: As Seryu was initially working as a member of the Imperial Police Force in the Capital, she was shown to have a more or less positive relationship with her co-workers. However, as seen after her successfully defeating Sheele and feeding her corpse to Koro, even the other Guards are unnerved by Seryu obvious psychological issues and instability.
Dr. Stylish: Ogre had introduced Seryu to Stylish and his experiments and was a very close associate of his due to that, even giving him a "Great Welcoming" by throwing rose petals when he introduced himself to the other members of the Jaegers. It is him who had modified Koro as well as given her new arms. For Seryu, her world had broken down when she was notified of his death. Like with Ogre, Seryu is noted to be completely oblivious and ignores Stylish's amoral and inhuman experimentation on other human beings and blindly hero worships him.
Esdeath: Seryu was shown to have a deep admiration and respect for Esdeath. After the death of Dr. Stylish, Seryu was promoted by Esdeath from a member of the Jägers to her own personal Subordinate. It was at that moment that Seryu saw her as attachment figure after the losses of her father, mentor and savior having sworn that her power and justice will all be for Esdeath. It is also worth noting that Seryu's last words before committing suicide via self-detonation were in memory of Esdeath being the only time she has shown a different priority than Justice, as upon remembering her she had a sudden change of heart of not wanting to die in the kind of place she did while tearing up.
Wave: The two are teammates of the same team, but Wave noted on how Seryu's view of justice was warped and didn't agree with it to the point of reporting it to Esdeath and describing her heart to be dangerous. However, after her and Koro's death, Wave, who was affected by the loss of his comrades, attempted to take revenge for her and Koro.
Kurome: Whenever Seryu and Kurome were seen together, they appeared to get along just fine, fighting well alongside each other and sometimes sharing sweets with each other. After their first meeting, Seryu even allowed Kurome to play with Koro.
Koro: Seryu is shown to have a positive and close relationship with her Biological Teigu, with her largely treating it like a close pet and carries him around everywhere and her lovingly naming it "Koro". As seen in her flashbacks, Seryu was the first and perhaps only candidates to bond with the Teigu, with it having violently slaughtered all other candidates prior to her. As seen in their interactions, she and Koro work well together as a tag team, with Koro himself seemingly sharing Seryu's bloodthirsty disposition on "Justice". Seryu herself was noted to enjoy overfeeding the Teigu with death row convicts or the corpses of her opponents.
As seen in their final moments together, Seryu cared greatly for Koro and was saddened by him having been damaged to the point that he could no longer regenerate. Later Seryu would hug him to comfort herself upon resorting to suicide bombing in last ditch attempt to kill Mine.
Mine: As it was Seryu who had killed Sheele, she had become the hated enemy of Mine who swore to avenge her comrade and friend. However, for Seryu, Mine was just another evil being that had to be punished yet escaped once. During their final confrontation, Mine expressed pity for Seryu when realizing how hopelessly misled and arrogant she is, and tries to elaborate on her childish worldviews and wrongful doings. To Mine's dismay, this only infuriated Seryu even more, not wanting to accept any pity or lectures from a "villain".
Tatsumi: Originally, she and Tatsumi got along well despite their short time spent together, with Seryu actually helping a lost Tatsumi. However, all of that changed when they began operating in enemy groups. Tatsumi held a grudge towards her for killing Sheele and had to hold back his anger when she petted him so as not to expose himself. Seryu initially remained unaware of his affiliation and treated him with warmth and affection following his ‘introduction’ into the Jaegers. When she learned where his loyalties lay, however, she vowed to kill him just for being a member of Night Raid. She never learned that Tatsumi was the one who killed her Mentor. Tatsumi also saved Mine from Seryu’s final attempt to kill her.