I don't think I've ever seen Chiyo smile so much and as happily as she did while she was with Ookuma. And then, when Igarashi showed up, she stops smiling and has a look of disgust on her face. And, of course, once again, she evades the whole truth and does not tell her "boyfriend" exactly what she was up to at 1:30 in the morning with another man. And, of course, she doesn't give a shit that he is not happy about her having a clandestine rendevous with another man. (I wonder who she was saying "Good night" to, her "boyfriend" or Ookuma?)
Chiyo sure seems to get along a lot better with Ookuma than she does with her "boyfriend." This is just the type of scene that pisses me off just about the most in manga. FMC has a private, intimate conversation with another man about her relationship with her "boyfriend", and especially a man who she knows is also after her affections. It is clear that Chiyo is not only lying to her "boyfriend," but to herself, as well. For someone who would "rather not see him again…", she seems perfectly comfortable being with Ookuma, chatting away, enjoying his company and talking about her most intimate thoughts. When has she ever had such an intimate conversation with her "boyfriend?" Never. Ookuma is now 2-0 over Igarashi with late night chats with Chiyo. It obviously doesn't occur to her that she ought to be spending those late-night times making special memories with her "boyfriend" (instead of another man), nor does it seem that she cares if he even finds out or how it will affect him that she is spending the wee hours of the morning with some other man. And why can't she be so open and honest with her "boyfriend" and tell him how she cherished her past with him? Why would she even talk about her cherished past with someone who caused her "love trauma?"
By the way, Ookuma, you may have asked Chiyo to stay with you until she fell asleep, but you did not force her to, she stayed because she wanted to. Plus, that doesn't negate the fact that she came seeking you out at 1:30am, in secret when everyone else was asleep, knowing full well that you two would be alone. And why did she have to do it in secret in the first place, and at 1:30 in the morning? It must have been because she actually wanted to spend time with you when no one else was around.
I certainly can't blame Igarashi for being pissed. I would be too if I discovered my girlfriend having a secret, intimate conversation with another man in the wee hourse of the morning when everyone else is supposedly asleep. A man who you all know is romantically interested in the woman in question. Since Igarashi now knows that there had been something between Chiyo and Ookuma in the past, and it seems like Chiyo is becoming interested in Ookuma again, it's a good time for him to either lay down the law with her, do not associate with Ookuma any more, or for him to call it quits on their relationship.