Ryu Katsunaga/Relationships

Unnamed Oldest Little Sister
The older one of Ryu's sisters is noted to be a very smart young woman and a college student. Ryu seems to have a good relationship with her.

Unnamed Younger Little Sister
The younger one of Ryu's sisters is noted to be quite ditzy. She and Ryu seem to have a good relationship as well.

Kota Sagano
When Kota met Ryu for the first time, he got the initial impression that Ryu was not his type of person. Despite this, the two young men ended up becoming close friends, as Kota found it easy to be around Ryu. They seem to share a lot of good memories from the times when they were still students at the police academy. They were roommates at the time. Once they were assigned their first posts, Kota cried a bit, having to separate from his friend.

Ryu got the news of him getting transferred to Oto in October, Kota's birthday month. The news of his transfer made him happy. Once he got there, he quickly got involved with Yui Sendo's case through Kota, after Yui's father kidnapped her and hurt Kako, who had tried to intervene to save Yui. Ryu worked alongside an unnamed female investigator, interviewing at least Kako, Heisuke and Akari.

Much later, after the case had been solved and Mr. Sendo taken to custody, Ryu had gone out to party and ended up so drunk he had had to stay at Kota's place. This is when he meets Kako's friends, Mikado and Jiro for the first time, as they had a study date planned with Kota for the day. Ryu is not hesitant to stay over and help Kota in tutoring the kids, however, he ends up being something of a nuisance to Kota, as he keeps flirting with Mikado and giving her love advice. Kota does not seem to think he is genuine in his approaches, calling it "harassment" and apologizing to Mikado for him.

Eventually, both Kota and Ryu end up getting kicked out of Kota's living room into the hallway with Jiro, as they start annoying Mikado with their antics. There, they take their time to listen to Jiro, as he tells them about Heisuke's aspirations to become a police officer, and how it inspires and motivates him to do his best as well. Ryu and Kota seem to share a moment of pride over Jiro's speech and tell him to study and work hard to reach his goals.

The same evening, Ryu forgot his wallet at Kota's place when he went to take Mikado and Jiro home, and has to return to get it. When Kota raises his suspicions of him having done it on purpose, he denies it, saying that he can read the room and knows Kota is rather slow in his approach (it is to be assumed they are talking about sex). This irks Kota, however, he is not fully offended by Ryu's comment, seeing as there seems to be a bit of truth in it. The two share a smoke with coffee outside on the apartment parking lot in celebration of the lovable group of youths they are now both tangled with.

Mikado Yaguchi
Ryu started flirting with Mikado pretty much from the moment he got to know her name. Whether that was him teasing her or feeling a genuine attraction to her is up to debate, however, when discussing the matter with Kota and Jiro, he did say he likes girls like Mikado. Ryu and Mikado's first proper conversation was about Mikado's heartbreak over Jiro, whom she in the conversations did not name. Ryu tries his best to cheer her up about it, saying that a girl as cute as her would definitely find a new man in no time, even going as far as telling her once she grows a little older she will have more than enough guys to choose from. He later on found out that the boy who had caused Mikado's heartbreak was Jiro, and was thoroughly shocked about the matter, implying that Mikado was too pretty for Jiro.

Later on, Mikado gives him Valentine's Chocolates, though she did not make them for him in specific. Regardless, Ryu is happy to receive them from her, and the two walk home together from the train station.

In Case. 62, he is shown coming to visit Mikado at her workplace before quickly leaving once he notices she had a customer, Yui. Mikado then has to explain to her friend that she and Ryu are not dating, that they never dated while she was in high school, and that they had simply exchanged numbers at Kako and Kota's babyshower. The final character pages in the same volume reveal that Mikado and Ryu get along very well, and that Mikado is not afraid to let Ryu fawn over her, so while it is clear he is still flirting with her, whether or not he is pursuing her seriously is not clear.

Jiro Nagakura
Ryu is met with compliments and admiration when he meets Jiro for the first time, however, these sweet words are immediately followed by Jiro asking him how many people he has had sex with, following a similar pattern as when Jiro met Kota for the first time. Ryu answers him by saying "[...] About 10.5," which leaves a lot up to the imagination. Once Ryu finds out that it was Jiro who broke Mikado's heart, he is so shocked, as he didn't have a high enough opinion of Jiro to be able to believe that someone as pretty as Mikado liked him.

Kako Motoya
Kako and Ryu have not been shown to interact much during the series. Kako was dying to make a good impression on him when they first met, but that went completely out of the window because of Mikado's ego and Jiro's weird, embarrassing sex-related questions. Moreover, Ryu's answer of having slept with 10.5 women sent shivers down Kako's spine, as the uncertainty of what he meant by this creeped her out. Luckily for her though, she quickly comes to realize that Ryu is a nice weirdo, and not bothered by his first encounter with her weird friends. Ryu knows of her and Kota being married and supports the relationship. He tries his best to give them as much space to be alone as he can.

Heisuke Okami
Pre Case 62. Ryu has been shown with Heisuke twice, both times for work. As such, how he would actually treat him is not clear, but he thinks of Heisuke as Kota's protégé, teasing the older one about it whenever he can find a chance to. There is a possibility that he is aware of Heisuke being Kota's rival in love.

In Case. 62 the two are shown sharing a conversation before Kako and Kota's wedding. There, he introduces Heisuke to his co-workers as Kota's protégé, teasing even Heisuke about the matter. The two seem to have a good relationship.