Jiro Nagakura/Relationships

Mr. Nagakura
Not much is known about Jiro's relationship with his father, however, from the interactions between them that have been shown, it can be deduced that they have a very comfortable relationship, as Jiro jokes about being a mixture of his parent's worst traits.

Mrs. Nagakura
Jiro's mother is never shown in the series, but his relationship with her can also be assumed to be good.

Ichika Nagakura
Jiro loves his little sister immensely. He is always shown speaking fondly of her, despite her tendency to diss him to make Heisuke look more capable than her brother. He is often seen carrying her around whenever she is with him, showing that he is very cuddly and protective of her. His care for Ichika is a point that he and Heisuke can relate to each other on, as Heisuke is also very caring of his step sister Yui. As a matter of fact, the two brothers use this feeling of care and worry as older brothers to convince Yui into letting Heisuke and his friends help Yui.

Jiro took Ichika with him to the Oto Cultural Festival and when she got lost in the school, he was extremely worried, practically crying by the point when Heisuke finally returned her to him.

Jiro has been shown to have a picture of Ichika holding Saburou as his profile picture on a messaging app.

Saburo
Saburou is Heisuke's cat. He saved it as a child from getting bullied by other kids, and he is shown to be very worried of the cat when it disappeared.

Unnamed Sons
In the last chapter of the manga, Jiro is shown to have two sons. His relationship with them seems to he good.

Fuko Onozuka
Fuko is Jiro's girlfriend.

The couple met for the first time at school, when Jiro happened to be passing by as Fuko was about to fall down a staircase. He was able to act quickly and cushion her fall, saving her from getting badly hurt. Sadly, he was wearing the bunny suit at the time, not giving Fuko anything to recognize him by, other than his voice. Much later on Fuko was getting bullied by some other students at Oto High, and Jiro managed to save her from the pinch again, this time showing her his face. This is also when Fuko started to develop a crush on him.

Whether or not Fuko is the mother of Jiro's two sons was not revealed.

Heisuke Okami
Jiro and Heisuke are best friends. Jiro wants Heisuke and Kako to start dating, as he is completely unaware of the fact that Kako is already married to Kota.

Mikado Yaguchi
Jiro and Mikado are childhood friends, and the two share a close, deep bond. They have known each other for most of their lives, and have always treasured one another.

At some point during the summer break of their second year in high school, Mikado noticed that her feelings toward Jiro had changed and that she had developed a crush on him. Her confession makes Jiro cry, as he feels guilty about causing pain for Mikado for rejecting her. Despite this, Mikado tells Jiro to go and confess to the girl he likes instead of worrying over her. The two continue being close friends even after this incident.

Kako Motoya
Jiro and Kako are friends with each other through Mikado.

Yui Sendo
Jiro cares a lot for Yui due to her being Heisuke's sister. He seems to trust Yui quite a bit, as evidenced by him leaving Ichika in her care, so the two girls could go out and make Valentine's Day chocolates with Kako and Mikado. That day Yui walks Ichika back home from Mikado's, explaining that Jiro had asked her to do so.

Kota Sagano
Jiro is not aware of Kota and Kako's marriage, thinking that Kota is only a distant relative of Kako's. He does not give Kota a good first impression, as he barges into Kako's home practically begging Kota to help him study for midterms after Heisuke had become unavailable. He then proceeds to tell Kota that his best subject in school is the 100-metre dash, which is not even a subject to begin with, and then he continues to ask Kota about his sex life when Kota was supposed to help him study. Obviously this annoys Kota, but Jiro somehow manages to squeeze a reluctant answer out of him (which is that he is not a virgin).

Jiro seems to make a habit out of bothering Kota in different ways. The most outlandish instance of all was when he, dressed head to toe in a bunny suit and troubled with love problems, dashed after Kota, who was running a lap around the neighbourhood, and forced him into helping him in his troubles. Jiro also drags Kota to come visit his family bathhouse. Despite Jiro being a nuisance, Kota still seems to care for him at least a little.

Jiro finally found out about Kako and Kota's marriage at their official marriage ceremony, at which point he noticed how obvious it had been that the two were dating all along.

Ryu Katsunaga
Ryu and Jiro met for the first time when Kako, Mikado and Jiro went over to Kota's apartment to study for an upcoming test. While there, Jiro initially made a good impression on Ryu by saying he had heard that Ryu was a really cool cop, however, he immediately follows this up by asking him how many women he has had sex with. Ryu's answer to this is even more bizarre, for he believes there to have been "[...] about 10.5."

Later on, during that same study date, Ryu and Mikado talk about her considerably recent heartbreak, which Jiro cannot help but overhear. He carefully asks about it when Ryu comes over to help him. This leads to Ryu being shocked to learn that it was indeed Jiro who had turned the poor girl down for another girl; he does seem to think Jiro is quite on-par with Mikado when it comes to looks (screaming, "YOUR FACE!" at him and then adding, "That FACE? Seriously?!"), indicating his belief that Mikado's feelings had been completely wasted on an average Joe, and directly after questioning Jiro's logic in rejecting someone like Mikado, making his fancy for her very clear for the ones in Kota's hallway. He does not tease Jiro for long though, and soon he and Kota encourage him to do his best and study well. Much later that day he offers a ride to Mikado and Jiro, taking them both home safely.

Mr. Motoya
Not much is known about his relationship with Kako's father and why it has ended up the way it is, however, Motoya does not seem to tolerate Jiro at all. Jiro has once commented on their relationship, stating that a visit to Kako's place when Motoya was home "[...] was like Hell on earth", and that Motoya just does not seem to get him. A reason for this could be that Motoya does not want Kako's male friends coming over (he seems to just barely tolerate Kota being in his house).