Per wiki policy, Spoilers Off applies here and all spoilers are unmarked.
Rei's relationship with the three sisters in general. Akari found him at his worst (abandoned on the street by a shogi opponent who forced him to get drunk) but quickly took him in to the Akamoto household, with everyone welcoming him as part of the family. They constantly care and worry about him, and he constantly tries to repay them for it.
Masachika Kouda is a terrible parent, but he saw that taking his deceased friend's son in is miles better than leaving him to suffer with his relatives who didn't have Rei's well-being in their picture.
Nikaidou's relentless support of Kiriyama, as a rival and a good friend in the game he loved.
He's the one who asked Shimada to lay a smackdown on his rival when Kiriyama ran into problems with Gotou because he thought Kiriyama was taking it way too personal. If that's not friendship then nothing is.
It may not be a huge moment, but when Momo thought Rei was her brother is also this.
Hanaoka and Nikaidou's relationship. Nikaido and his family sent the loyal butler to a 80 day long trip but Hanaoka came back because, being away, he was too worried about the Ill Boy he's been looking after since childhood. When he gets back and finds Nikaidou indeed in quite a state, he wastes no time in taking him to the doctor (indeed he called the doctor previously, correctly thinking that without him around Nikaidou may slack off on the diet and pills he needs for his health).
When the Kawamotos' grandpa fiercely tells Hina that even though she became a victim of bullying because of it, defending Chiho was the right thing to do.
Kiriyama's hurrying to Kyoto just after winning his match against Junkei in Chapter 63, because he remembered that Hina got stomachache the day before she go to school's field trip. One might see Kiriyama as creepy stalker, but we're talking about a guy who, at the start of the series, had no idea what social life is. Running all alone in a city he barely set foot in to find a person he cared for is impressive gesture coming from him.
Special note goes to Chapter 139, where she finally has her Heel Realization and, no matter how much it hurts her to do so, pledges to stay out of Rei's life from here on out while she works on resolving her own issues, and sincerely wishing for him to continue finding happiness in his life with the Kawamotos.