Concern
How Hiro worries for Rin.
As one of the youngest of the current generation of juunishi, Hiro is possessed with the spirit of the sheep. He has feelings for Kisa and approached Akito about it. Akito was furious when hearing this and to punish Hiro, she gave Kisa a thorough beating that resulted in her being admitted to the hospital. Hiro felt guilty about this, feeling it was his fault and decided to try to ignore Kisa so she wouldn't have to suffer from Akito's wrath. Later, he would witness the incident in which Akito threw Rin out of the window.
As the only person to know what truly happened to Rin, Hiro frequently visits Rin in the hospital and worries about her welfare. He also appears to be the only one who knows of Rin's plans to break the curse, which makes him worry even more about her safety.
After not seeing Rin for some time, Hiro notices her walking home from school and runs to talk to her alone. He notices how pale she is and as he worries about her health, Rin tells him to stop worrying; he has parents who love him and he should be happy for it. Hiro can't help but notice her attitude, commenting that it seems as if Rin thinks it's wrong to have no doubts about his parents love for him. He also seems afraid that Rin hasn't found any way to break the curse.
Hiro is also one of the few to notice Rin's disappearance. When he hasn't seen or heard from her in a while, he visits Kagura's home to ask if they know anything about where she is. They don't and Hiro returns occasionally to ask for any updates. Eventually, he runs into Hatsuharu and tells him that if he's looking for Rin, she hasn't been home for some time. Significantly worried about her, Hiro confesses to Hatsuharu that the scar on Rin's back wasn't the result of an accident. He tells Haru about Akito's assault and that Rin broke up with him so she could protect him while she looked for a way to break the curse. Hiro begs Haru to find Rin and tell her to give up on her quest so that she no longer has to suffer alone.
Analysis
My opinion on their friendship.
Despite Hiro being one of the few people close enough to Rin to know about her plans, we rarely see them interacting with one another. Hiro frequently worries about Rin's health, perhaps feeling guilty that he was unable to do anything to prevent her from being pushed out of the window. He's the type of boy who wishes he could be older and more mature in order to take on more responsibility.Seeing Rin being pushed out of the window was likely a traumatic experience for Hiro. As his own relationship with Kisa reflects Rin's relationship with Hatsuharu, he likely imagined what would happen if their roles were switched. Picturing Kisa being thrown from the window is likely something that haunts him, which makes him worry about Rin all the more.
Hiro's concern for Rin does seem genuine as he's one of the very few who bothers to try to find out where Rin is when Akito has her locked away. No one other than Tohru seems to care enough to actually notice Rin is missing, let alone look for her. Hatsuharu doesn't look because he began to believe that Rin really had lost interest in being with him but is encouraged to find her when Hiro confesses the truth.
Rin seems to generally push Hiro away and act coldly towards his concern. She likely sees him to be as kind and sweet as Tohru and doesn't want him to get caught up with what she's doing in case Akito finds out. Both Rin and Hiro tend to want to protect those around them. It's a little disappointing that we don't get to see Hiro's reaction to hearing that Rin was confined or his reaction to seeing her afterwards. I like to imagine that Hiro continues to check up on Rin for some time to make sure she's alright and as Rin begins to heal, becomes more reciprocative of his concern.