the knight is Michael, also known as friend.

michael was a "big shot" thanks to an unhealthy and secretive relationship with december, and his life crashed and burned when she went missing.
his main priority is finding december, because of a delusional belief that it will somehow turn his life around and make him successful again
michael specifically blames asgore for everything that happened, which is why the knight is going to be violently targeting flower king in chapter 5.