Unmarked spoilers are ahead, per Spoilers Off rule.
While Reiji acts annoyed by him most of the time, it’s clear that he values Takamiya as a dear friend and cares deeply about his happiness and wellbeing. His criticism and derision of Takamiya’s relationship with Izumi is because of the fact that he doesn’t believe the relationship is good for Takamiya. He ultimately realizes how much Izumi loves Takamiya when Izumi reaches out to Reiji for help with Takamiya, and after this he gives them his full support.
After Naoya moves out in volume 2, Reiji sneaks out of the hospital with broken ribs to look for Naoya on Christmas (also the latter’s birthday). When he finds Naoya, alone with a Christmas/birthday cake, Reiji declares that he “hates brats” but Naoya is different and he wants Naoya to come back and stay by his side. Naoya cries but happily accepts with a kiss.
While celebrating New Years with the Aoe family, Naoya finds a stray kitten and he spends a lot of time outside in the cold tending to it. Though he feels he has no right to ask Reiji to keep it, when the latter finds him he picks up the kitten and puts it in Naoya’s arms, showing that it’s okay for him to have the kitten. He later sheds Tears of Joy when the family fusses over him and he relishes celebrating a holiday with loved ones again.
When Haruomi is “adopted” into the Kashima family, Shuhei immediately accepts him as his big brother. While it takes Haruomi a bit to warm up to Shuhei (because he feels guilty for leaving his twin behind) he ultimately embraces his new role in Shuhei’s family after he realizes how much Shuhei cares for him and how Haruomi’s aloofness is hurting him. Years later they still maintain a very close relationship as brothers.
Takamiya and Izumi’s relationship has plenty of heartwarming moments.
When Reiji tricks Izumi into eating aphrodisiac-laced chocolates, Takamiya decides to leave the apartment so Izumi can recover by himself without worry. When Izumi questions why he wouldn’t try anything, Takamiya makes it clear that he won’t do anything if Izumi didn’t want it. Izumi has a Love Epiphany in that moment and hugs Takamiya, making it clear he wants to be with him.
Izumi trying everything he can to get to Takamiya’s apartment in time to celebrate Christmas with him. Facing crises and weather delays, he ends up running in the snow to get to his apartment, but ultimately arrives too late for Christmas. Takamiya however is so moved by the lengths Izumi went to get to him that Takamiya doesn’t mind in the slightest that the holiday had passed.
Izumi finally telling Takamiya that he loves him. And then in a later chapter when Takamiya falls into a brief funk Izumi remains by his side and assures him that he will stay by his side until the very end.
Kiichi’s relationship with Naoya is plenty heartwarming, with the way Kiichi dotes on Naoya like a proud big brother. And when Naoya protests being spoiled by Kiichi the latter will hear none of it and insists that Naoya only responds “Thank you, Sensei” to the nice things he does for Naoya.
After Yanase Izumi revealed his past and confessed his profession to Arashi, to Izumi’s shock Arashi accepts him unequivocally and asks Izumi to choose him and fall in love with him. Izumi is reduced to tears in Arashi’s arms, and after a period of personal conflict and believing himself to be unworthy of Arashi, in the end Izumi does choose Arashi, quits Blue Boy, and leaves his past behind once and for all.
Wataru spending all night digging through his estate’s massive garden to find Naoya’s house key so that he can go home to Reiji. Naoya cries and hugs him, leaving the poor boy confused that Naoya “wasn’t happy” about him finding the key.
Twice when Naoya gives Reiji a gift; a tie for New Years (before they become a couple) and cologne for his birthday (after they become a couple) he is very insecure and self-deprecating about himself when he presents the gifts. Reiji acts annoyed about the tie but he wears it for the New Years celebrations. With the cologne he has a more gentle reaction, testing it on Naoya and deciding that he likes the scent. Reiji ultimately ends up switching to this cologne permanently from his previous preferred brand.
Seiichi and Katsuki’s friendship deserves mention. Seiichi tries to discourage Katsuki from pursuing Jin because he does not believe it will end well for Katsuki, but ultimately he gives Katsuki his support and asks Jin to not hurt his friend.
