@homeworkChan you are looking at him from a POV of a normal person. Do you remember what he went thru to his entire life and even now that they just escaped. He didn't know much about love and now that he began to understand he doesn't want to share it. His mental capacity with respect to the emotion department is 5-6 years old. They are selfish.
That is why later one he became normal after meeting the heroine. She little by little taught and showed him to understand that she is not going away. She will need to do the same thing in her own way but will need to do the same. teach him and show him what romantic love and family love is and that she won't go away.
I get it that you would never do that with a guy you meet. You will want a guy with a mental maturity in the love department of a grown-up man, but she is starting to like a person that doesn't understand much. Since she likes him and looks like she wants to be with him, it is a problem she will need to toss to her shoulder and take responsibility. She doesn't need to nor have the obligation, BUT she wants it.