Rhea of Thorolund is the daughter of a prestigious family in the land of Thorolund. She is a priestess in the Way of White and is on a pilgrimage, accompanied by her two former classmates and current bodyguards, the Clerics Vince and Nico of Thorolund. Rhea never reveals the details of her mission, but speaking with Griggs of Vinheim before she leaves Firelink Shrine will reveal that she is actually after the Rite of Kindling. Knight Lautrec of Carim will also reveal, for a sum of Souls, that her followers have been killed and that she is trapped below, in the Tomb of the Giants.
Rhea first appears next to Petrus in Firelink Shrine after the Capra Demon is defeated, where they remain until their dialogue has been exhausted or Pinwheel is defeated in the Catacombs. She then winds up in the pit where Patches tricks the player. There, she requests the Undead to "free" her escorts. After killing both of them, she will give the Replenishment Miracle as a reward and then move to the Undead Parish.
If the player has received Lautrec's tip and spoke to Petrus, the latter will reveal his true nature and threaten Rhea's life. Then, if the player doesn't immediately go to the Undead Parish, there will be only severn Humanities and a Pendant in place of Rhea, in front of the altar. If Petrus is then killed, he will drop an Ivory Talisman which is unique to Rhea.
If the player goes straight to the Parish after rescuing Rhea or have not received Lautrec's tip, she can be found in front of the altar selling miracles. She will reveal that it was Patches who had trapped herself, Vince, and Nico in the pit in the Tomb of the Giants. Petrus had abandoned them, but she does not seem to hold a grudge against him for this action.
If Petrus is prevented from killing her and all her miracles are purchased, she will go Hollow and can be found in a cell in the prison tower of The Duke's Archives.