Responses
Naoto, at some times, were being possessed by the older brother. That's it.
But I wonder why did he just disappear. He could have showed up when they found Kageyama at least once
Hmm, he was only used as a tool for Kageyama to express his feelings (although extremely unconventional) to the MC. I don't see any reason for him to show up again, but I do understand your attachment to him as a character.
What about brother? The way he was acting was weird? I really thought he was Kageyama who pretend to be Naoto, or that he is dead and Naoto is possessed with his spirit