Petrus is the blonde Priest/Cleric NPC who is waiting for Princess Rhea of Thorolund and her two school friends/bodyguards at Firelink Shrine at the beginning of the game. He gives you the Copper Coin as a token of good will, offers to teach you minor miracles, sell some talismans, and teaches you a gesture. You can also enter his covenant, the Way of White, by speaking to him.
After ringing the first bell of awakening and beating Capra Demon, he is joined by his companions again [Vince, Nico and Rhea of Thorolund] as they plan to set off on their secret mission into the Catacombs. If you save and quit, all of them will be gone, but upon saving and quitting again, Petrus will have returned quite distraught, claiming to have lost his lady. If you talk to him before entering the Depths there will be an event with him, Lautrec of Carim, and the full story as to why he is once again alone after entering the catacombs with his companions. He still teaches you miracles and such, but is much less talkative.
Encountered after heading up the stairs in Firelink Shrine towards the elevator leading to the church in Undead Parish just before fighting the gargoyles. (You must take the elevator down from the church first to unlock the elevator for traveling between Firelink Shrine and Undead Parish)