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
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.
There are few pages missing from chapter one