Rhea is a priestess in the Way of White and the youngest daughter of the House of Thorolund. After becoming branded with the Darksign and turning undead, she is sent by the Way of White on a quest to Lordran to retrieve the Rite of Kindling. Two clerics, Vince and Nico, serve as her companions and bodyguards on the journey.
Rhea, along with her companions, will appear at Firelink Shrine near Petrus after the player defeats the Capra Demon in the Depths. She will remain at Firelink Shrine until the player either listens to all of her dialogue or defeats the Pinwheel boss in the Catacombs, at which point she and her companions can no longer be found at the Shrine.
Later, while exploring the Tomb of the Giants, the player may come across a treacherous adventurer named Patches. Patches will trick the player into falling down into a large pit. Rhea and her companions have suffered the same fate, and she will request that her companions, who have gone completely hollow and insane, be put out of their misery. Doing so and then speaking to her again will free Rhea from the pit, and she will then travel to the church in the Undead Parish.
At this point, her story has two possible outcomes.
If the player immediately travels to the Undead Parish, Rhea will be found praying at the altar in the church. She will reveal that Petrus abandoned her, along with Vince and Nico, after being tricked by Patches. She will also sell the player miracles. When questioned, Petrus will threaten to kill Rhea and after an indeterminate amount of time will do so. Rhea's corpse can be found in the Undead Parish church and can be looted for seven Humanity and a Pendant. Killing Petrus at this point will cause him to drop an Ivory Talisman, an item unique to Rhea, removing any doubt of his involvement in her death. Killing Petrus before he makes good on his threat is the only way to prevent Rhea's death.
If the player does not immediately travel to the Undead Parish church, Petrus will murder her before the player has a chance to intervene. Her corpse and items will appear as described in the alternate scenario, and Petrus will again drop Ivory Talisman when killed.
If Rhea is saved and the player has purchased all of her items, she will eventually be captured by the Channeller watching over the Undead Parish church. She can then be found, hollowed and insane, in a cell in the prison section of the Duke's Archives.