The big guy in metal armor… ?

This is my second playthrough. The first one I did not go much further than the forest with the bush guys and the big ass soldiers, but I realised I missed plenty of things.

I started my game with the Master Key and I’m currently at the Undead Burg, and right before the dragon boss area there is a door to unlock and you go down two or three floors. And THEN, there is a fat guy in armor chasing you. I obviously ran away because I did not want to lose all my souls… but I wonder who that is and if I could come back later for it because he seems impossible to beat. Can I even open the door behind him without having to kill him ? ;-;