The bobbed hairstyle says: Neatly trimmed, as the old nobility
Gold hair/eyes beloved by certain members of the nobility
The knight shield also states: Although it is thought to have been used by ancient knights of the nobility, its defense capabilities are similar to other shields.
Elite Cleric Set says: those that were black belonged to warrior clerics sent on missions, many of whom were high-level priests.
Sorry if this has been discussed before but is it possible Petrus is a member of the royal family in Thorolund? I get the feeling he may be someones bastard child that can't officially inherit. Instead he was made a high level priest but the father/mother wanted him out of the way so they sent him to Lordran. He then kills Rhea either out of jealously that she was formally acknowledged as a princess or because his father uses him to kill all undead members of the Way of White that are too important to simply send to the asylum.
Also the purple hair colour says it belongs to a near extinct race. I can't remember anyone in game with purple hair but a lot of people wear helmets. So far Zena comes to mind but if anyone else has a better idea I'd love to hear it.