Not that it's bad, but it's a little obvious, isn't it? That the real identity of the mystery character is the only main character who went missing a while ago.
This really didn't need to be an isekai. Having read up to chapter 17 there has been only two times, other than this first chapter, that it has been briefly brought up.
He could just as easily have been an elf who got bored with elf life.