Glitch in the System (Male Reader x Sword Art Online)

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 coul...