Otani going to the Umibozu concert with Risa instead of meeting up with his ex-girlfriend. Made even more sweet when Risa is comforted by Otani's presence.
Episode 18, specifically Koizumi and Otani's "real" kiss, and Episode 19, where Otani confirms he likes Koizumi (on a giant stadium screen, too). Oh, and he calls her his girlfriend for the first time, without batting an eyelid. Cute!
Nobu comforting Risa after being rejected by Otani the first time.