Surprisingly, the flashback of how Sumika and Nanami met each other.
The chapter where Manabu recalls a memory of him witnessing his grandpa help out a woman in an abusive relationship after she comes to their house for help. The woman didn't even really know him and only came to him for help because she knew of him from other people, yet he still helps her out without a word of protest. It really makes you understand why Manabu has such an enormous amount of respect for his grandpa.
The lengths that Sumika goes to snap Nanami out of her Heroic BSOD due to being rejected by Manabu. She's willing to go to Reina, who she's not fond of, tell her about what's happening to Nanami, and asks Reina not to break up with Manabu so Nanami can have him like you'd expect from someone like her, but begs her for her help.
What snaps Nanami out of her funk — seeing Reina roughing up Sumika, she fights the former not because it's their usual routine, but in order to protect the latter — just like she used to do when they were younger. The flashback showed how much the two cared about each other, this chapter just served to cement it.