Katya initially agrees to a marriage with Koito, despite not being thrilled about it, to please her father. She knows Koito and finds him to be lively and dedicated, yet foppish and immature. She resolves to be happy with the match - after all, it is someone she knows (and finds handsome), and surely her father believes this is the best match for her and it would be rude and disgraceful to decline it.

Koito, for his part, is absolutely smitten with Katya right away. Why wouldn't he be? She is beautiful and gentle, and of course, the daughter of his idol, Tsurumi, and this must mean Tsurumi values him enough to want him to be his family - oh joy! Koito tries to impress Katya with boasts of his fighting prowess and shows of strength. Katya, for her part, is polite but unmoved.

Katya is sent with the Karafuto group to Russia to retrieve Asirpa to act as an additional translator and a 'softening' presence for the group. Koito is directed by Tsurumi to protect Katya, an order he takes seriously, as not only does he want to impress Tsurumi, he wants to show Katya that he can care for her as a husband. throughout the journey across Karafuto, Koito does his best to make the journey comfortable for Katya and to protect her from man and beast alike. Katya begins to fall for Koito, realizing that Koito's affection for her may not simply be an extension of his affection for her father. If he didn't care about her personally, he wouldn't be hovering around her as she cooks, wrapping his coat around her, etc?

At the confrontation to retrieve Asirpa, Katya recognizes Kiroranke as part of the group that killed Fina and Olga. In her grief, Katya attempts to wrestle Tsukishima's rifle from him. Kiroranke quips that Katya should know better than to point a rifle at someone, lest accidents happen. Tsukishima is able to knock Katya back. Koito confronts Kiroranke, telling him to leave Katya out of this and to focus on him right now, despite his curiousity about the connection between them. Katya is impressed by Koito's display of power and touched that he would come to her defense and personally rebut someone she holds genuine fear and hostility towards.

At the hospital, Katya is commissioned to assist in the OR as both a nurse and translator for Ogata. When Ogata escapes, the first thing Koito does is run to the OR to ensure Katya is okay. That he would run headlong into danger for her touches Katya deeply. After Ogata escapes, Koito and Katya rush to each other to ensure the other is okay.

After Koito asks Tsuksihima and Katya what 'barchonok' means at the port and begins to verbalize the pieces he is putting together about Tsurumi, Katya provides a bare backstory as to how she knows Kiroranke but that it’s too painful to discuss and she is rattled by having seen Kiroranke after so long. She mentions Fina and Olga by name, but not their relation to her. Koito, though somewhat dissatsifed and wanting to satiate his own curiosity, states that he would be happy to talk about them with her since talking about the deceased keeps them alive. This is essentially what makes Katya go “Oh, this is my person. He’s grown so much from the excitable young man and he’s principled and caring.” She kisses him on the cheek and says that she appreciates that offer immensely and she’d love to know more about his brother as well. (Tsukishima sends Katya away so that he can threaten Koito to not disrupt Tsurumi's plans, lest he want Tsukishima to make Katya a widow before she's even been married).

After being stabbed by Sugimoto, Koito is sent to the hospital to recover. Katya takes it upon herself to personally care for him, physically and spiritually. (She wishes Koito wasn't so interested in Inkarmat's fortune-telling though...hmph!)

The fight between Tsukishima and Tanigaki erupts out of the hospital with Tanigaki's escape thanks to Ienaga. Koito takes Katya with him in pursuit, believing she can assist with the birth. Once they arrive, Koito orders Tsukishima to stand down and they are all hastily assigned to assist with the birth. The next day, Koito allows Tanigaki, Inkarmat, and their baby to escape. Katya is amazed at how much Koito has grown since they met. She tells Koito that he is a remarkable person with a noble heart, and she is glad he let Tanigaki go be free from this mess with his family. Resolved in her feelings for him now after having seen him grow, Katya tells Koito that she loves him for the first time. Koito is ecstatic at hearing that from Katya, states he is grateful Tsurumi believed he would be a good partner for his daughter (he this trying to believe in Tsurumi, and believes Katya is a pawn to Tsurumi like himself) and that he loves her too (and then they kiss...obviously ;) )

Sapporo Beer Arc - Katya is present but uninvolved in the action. Once at the church, the connection between Tsurumi, Katya, Sofia, and Kiroranke is revealed to the eavesdropping Koito and Tsukishima - Katya is the biological sister of Tsurumi's wife, Fina, and aunt to his daughter, Olga, who were killed by Wilk's group. Tsurumi took Katya with him back to Japan, knowing that he could use the only person in the world who knew what happened for his own gain.

After the reveal, Katya goes into the back where Koito and Tsukishima were hiding. Realizing they heard everything, Katya apologizes for not divulging the full story sooner, and that it wasn't just hers to share. She is glad that Koito knows now. He says that she should tell him more about them later.

Final battle - essentially the same. Koito resolves to beat Hijikata for everyone's sake but imagines Katya waiting for him. Katya waits in a nearby church praying for everyone's safety.

After the battle, Koito goes to collect her and informs her of Tsurumi's apparent death. Katya is deeply saddened at the loss of the man who raised her but hopes he is at peace and laments that Tsurumi loved his family so deeply that it warped into hate and malice. She hopes that his love for his fmaily may outwreigh his sins and allow him into heaven with Fina and Olga, and if he is not, she prays that they will protect his soul.

Koito tells Katya that he is now de facto leader of the 7th and his priority is now protecting them from prosecution, and that includes her. Koito insists that they marry right away - both in order to protect her and because he loves her. He wants to repair the 7th from the harm Tsurumi caused and can't imagine how to begin without her by his side as his wife. Koito adds that although Tsurumi has done many horrible things, he will always be grateful he gave Katya to him. Katya accepts his offer to marry immediately, and they resolve not only to rebuild the 7th, but to build a family of their own and start a new chapter in their lives <3

Post canon, Katya no longer practices as a nurse but is a philanthropist focusing on causes to support war orphans and military veterans. Koito and Katya have two children - a daughter, Yuliya, and a son, Konstantin.