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.
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.
Esdeath: Seryu was shown multiple times to be infatuated with Esdeath or perhaps even had romantic feelings for her. 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 by exploding 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 view of things 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.
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 mislead 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, however all of that changed when both learned that they operated in enemy groups. Tatsumi held a grudge towards her for killing Sheele and had to hold back his anger not to expose himself. Seryu vowed to kill him just for being a member of Night Raid to avenge Ogre. She never learned that Tatsumi was the one who killed her Mentor.