Oceiros is a 2v1 fight?

During one of my runs I was fighting Oceiros. I noticed that the knight guarding the boss fight was standing still, blocked off by the fog but somehow on the wrong side of the fog.

When I got close he decided to join the fun, I still won but I dropped down to 1 estus before I managed it (which is not very good considering Oceiros was 90% dead when this happened).

Joining the fun

Me on the floor crying