The Hidden Boss is too mysterious

Artorias is a strange man. Sometimes he acts like he is a wallpaper in a hall full of people , readily accepting insults from his classmates while maintaining a poker face while sometimes he openly shows how strong he is despite being inferior in social status.
Who is he?? He was not mentioned in the game, nor did this type of character mentioned to us by the game characters. Is he a hidden boss?? or he is just a mob??
Find out how this mystery person appears in front of the 'original ' protagonists in the game and how his actions change the world derived from a game made in Earth.