Haru and Subaru's relationship in general. While turbulent at the start, they gradually start to care for each other and bond over their differences. Along with that, it's heavily implied that their relationship allowed them to heal from their past wounds, which allowed them to be much better people (animals?) in the end.
Kawase is a genuinely good guy throughout the series, going out of his way to try and take care of Subaru, starting from episode 1 when Subaru had passed out from exhaustion and hunger and immediately came over to check on him. From there Kawase becomes a close friend and supporter.
In episode 3, Subaru realizes how much effort his parents put into making trips that he'd have enjoyed, had he gone with them. When he breaks down in tears at the realization, Haru comforts him. Haru acts up and is attacking something Subaru can't see. When it swaps to Haru's point of view, it turns out to be the spirits of Subaru's parents who were worried for him and thank Haru for being there with him.
Subaru's Character Development. Starting out as an antisocial recluse, he gradually learns how to let people in, which in turn makes him a much more friendly person in the end. He still has his issues to work out, but it's shown that his new experiences allowed him to finally start healing from the pain he's felt for months.
Haru reuniting with Hachi. After it was shown that one of Haru's remaining siblings was killed by crows, seeing her reunite with her brother is so goshdarn heartwarming to see. Hachi introduces Haru to Roku, another cat owned by Nana, and while Haru is prepared to be attacked for “being in another cat’s territory”, Roku immediately tells Haru she’s in a safe place with nice humans and that he understands that she’s probably been through a lot of hardship on the streets, being a former stray himself. Roku even goes as far as to say they’re family, being that Haru is Hachi’s sister and he’s Hachi adopted brother.
Episode 6 has Subaru get extra flustered finding out that not only does Nana and his younger brother Yugo love books, Yugo is specifically a fan of HIS novels, especially the one he’s been writing based of Haru.
Subaru becomes even more flustered over the amount of fanmail he gets, realizing he never really considered how many people love his work. And among the letters is one from Yugo as well.
When Haru was a stray, there was a little girl named Haru who would feed her from time to time, to the point where Haru (the cat) would associate that name with “food”, which is why she chose “Haru” as her name from Subaru. Later on, when Kawase stops by and Hiroto comes over with his siblings, they order takeout from the restaurant where Haru (the girl) works with her mother and she’s happy to hear that the once stray cat who stopped coming by now has a home and that they have the same name.
Subaru and Haru reuniting with each other in episode 12. Words simply do not do enough justice into explaining how truly heartwarming the scene is. Doubles as a Funny Moment due to Subaru getting stuck in the gutter after trying to pick up Haru.
