Meet the Gang!

Who is Y/n? Why does he keep appearing in the dungeons or missions that Kirito and his friends explore? Why is it that Y/N can see memories of a girl that talks to him so quietly yet so softly? Why does he show interests in the gamer Sinon? But most importantly, why is it that Y/N knows who Kirito is? I mean, he could just be a glitch in the system, right? (ANY PICTURES IN THIS STORY DO NOT BELONG TO ME AND BELONG TO THEIR ORIGINAL OWNERS)