She will mention that she and her followers are on a mission, but never reveals the details. Talking with Griggs of Vinheim, before she leaves, will reveal that she is actually after the Rite of Kindling. Knight Lautrec of Carim will reveal, for a sum of Souls, that her followers have been killed and that she is trapped.
If Petrus is still alive at this point, he will soon try to assassinate Reah. If not, Reah will remain in the Parish until the player exhausts her stock of miracles, whereupon she will travel to the Duke's Archives and become hollowed there.
Reah sells Wrath of the Gods which can only be bought from her. She also sells basic miracles at a better price than what Petrus offers. If killed, her Ivory Talisman is one of the best Talismans in the game; although it still falls short in terms of power to the Darkmoon Talisman, it is much more easily obtained.
After Petrus has killed her, he will drop the Ivory Talisman when killed.
12 humanity can be drained from her using the Dark Hand.
As of Patch 1.05, there is a glitch where Petrus will not appear at Firelink if players proceed directly to the Tomb of the Giants without talking to Reah and her group at Firelink Shrine and rescue Reah at the Tomb of the Giants. The unfortunate result will make it impossible to save Reah from her demise at Petrus' hands.
You are Undead, as well? Then we've no time to fraternize. I have my mission, and you no doubt have yours. We must not let this curse overcome us.
Speaking with her at Firelink
Did I not explain the urgency of our tasks? Who are you so uncouth as to lack such judgement? By the looks of you...I should think not.
Finding her in Tomb of the Giants
You're no Hollow, are you? Thank goodness. Please be careful. There are two fierce Hollows not far from here. They were once brave knights... my former escorts. Who would let such strong spirits be Hollowed so? Heavens... Is there nothing... Nothing at all to be done.
Speaking to her after killing Hollowed Vince and Nico
You banished those two Hollows did you? It pains me to think of the trouble my failings have caused. I am certain both Vince and Nico are grateful to you. Thank you so very much. Here, these belonged to them. You deserve them more than I.
Speaking to her in Undead Parish
Hello... I will never forget what you did. I am deeply indebted, for it was not within my power to save Vince or Nico. I cannot thank you enough. In case you have not heard, I am Reah of Thorolund. I only wish there were some way I could help you, but I am inexperienced, and I only know the art of Miracles. If that could be of any help, speak to me again.
Speaking to her again
Would a Miracle be of any help to you? I would be most pleased if that were so. May we discuss Miracles, then?
Vince and Nico were fooled by a lout named Patches, and turned into Hollows. My prayers did them no good. It is my ignorance, my frailty that has sealed their fates. Perhaps Petrus realized my weakness all along, and thus made the decision to abandon me. I can hardly blame him now.
I do not warm easily to unfamiliar faces. The bonfire below is so frequented, it makes it difficult... I have lost all those who were close to me.
I was wondering when you might return. It is a great relief to see you. Now, let us speak of Miracles.
I am most pleased to find you in good health. I am ready. Let us speak of Miracles.
Oh! Please, forgive me. I was absorbed in prayer. The Miracles, I presume? I will be ready immediately.
Then, be safe. Farewell. Vereor Nox.
Ow! Horrors! The nerve! Are you a heretic, or just plain Hollow? It matters not, for you shall not escape! You will offend the Gods no longer!
Why on earth would you... Perhaps this is my punishment? Clearly, I am no threat to you. But please, I ask just one thing. If I do go Hollow, then finish me off. I beg of you.*