Chisaki, the typical tsundere girl, but cold. It's obvious that Chisaki has feelings for Ueno-kun; it's not just masochism or that he's the only one she feels for him. There are deeper feelings, which is why she let Ueno-kun go so he could find happiness with Manaka. Chisaki let Ueno-kun go so he could be happy because she truly feels something for him. That's why Ueno-kun in previous chapters seems a little sad or discouraged that she sacrificed her desire for him so he could find happiness, but that's not the case. It's the kind of cliché where they shouldn't be together, but fate wants them to be, even though they don't want to be. Besides, Ueno-kun and Manaka seem like a couple who want to get off drugs, and they do, but they only end up as friends who got off drugs. They're so focused on leaving the past behind that they forget they're a couple. When they break free from that programming, they'll be fine, but when they want to be together, they won't know what to do. Manaka can fight, but I don't think she'll do what Chisaki or Ueno-kun did to her. Makana and Ueno-kun don't really like each other; they just want to break the programming, that's all. That's why Chisaki said he was going to find someone like himself to be his slave for life—that's what happens when you get married—and she left the door open for Ueno-kun if he's interested in coming back.
Ueno-kun realized that Chisaki had found happiness with him and was happy only with him, When Chisaki thought she was having sex with Ueno-kun, she felt happy and free.