Chizu’s dilemma seems to be her processing the possibility that she doesn’t know everything about Ryu. She knows everything about him and already considers herself the person closest to him. Romance hasn’t been in their conversations except when it came to topic about his brother. So now she is thinking about Ryu’s side about love because that was something she never considered. Then made her realize that it was a side of him she never knew. Being told by Ryu that’s a confession is not needed for her to know and to find out he likes someone else while not knowing about it really affected her. Then the next thought comes to mind, “will there be a time he goes out with someone without her knowing?” “Will that person be the one closest to him, not her?” Which can explain her defensiveness to the girl trying to separate them.
She states that she doesn’t see Ryu that way. But I would imagine the next question for her will be “Can I imagine myself accepting Ryu moving on from me to someone else?”