All of chapter 12. Starting with Daikichi bringing up the idea of legally adopting Rin: Rin listens to Daikichi's idea of becoming "Rin Kawachi", and refuses, insisting on keeping her father's name Kaga. Daikichi is moved to tears when Rin proclaims "Daikichi, I like you just being my Daikichi." Later, Daikichi assures Rin that just because she's entering first grade doesn't mean she has to give up things like sleeping next to Daikichi or having him hold her, promising "I'll hold you even when you become an old granny, if that's what you want." Rin starts crying, but quickly breaks away from Daikichi's subsequent hug, and apologizes for not hugging him when he cried earlier and promising she'll give him a big hug next time he cries.
At the funeral when everyone is putting Chrysanthemums on her father as a final goodbye she refuses and runs off like Reina had done moments before, but she then picks two bluebells, his favourite flower, and places them on him instead.
Rin's cat-teeth grin (final photo) in the last episode gave me the wibbles because of the sheer cuteness.
The earliest, of course, is when Daikichi calls out his entire family over their callous behavior towards Rin and deciding to raise her. Unlike most instances of sudden parenthood, he wasn't under any legal obligation to care for the child, and he certainly wasn't under any family pressure to take care of her (it's strongly suggested that much of his family would have encouraged him to not take Rin in if Daikichi gave them the opportunity to object). Daikichi had a choice, and he chose Rin.