Marjolaine is a character from Leliana's past. She is described in detail by Leliana if you strike up a conversation about Leliana's past after your approval rating with her is high enough.
You ultimately have the choice of either attacking Marjolaine or telling her to leave. If you tell her to leave, you will still get some good items, located in the chest in the bedroom (including Marjolaine's Recurve).
Marjolaine hits VERY hard for a rogue, even if not in a position to backstab, so try to limit her damage in any way possible. Penalizing her attack through skills such as War Cry or Crippling Shot is very effective, as is boosting your tank's defense - Heroic Defense and Glyph of Warding are excellent choices.
Aside from Marjolaine herself, she also brings along several normal rank enemies, including heavily armored guards that are a nuisance, but no huge threat, and two mages. These enemies pour from both sides of the room at the initiation of combat. Have your tank gain Threat - Taunt works well - then quickly dispatch the mages, as their spells buff Marjolaine and can wreak havoc on your party. However, note that there are traps in front of each door that the mages stand in.
A good strategy is to retreat to the room where you fought the first two guards. This causes the mages to follow you, and gets all your targets in a tightly knit group where they are more vulnerable to spells and abilities such as Chain Lightning and Scattershot. Often Marjolaine will simply activate the level 4 Bard talent: Captivating Song, and let her guards do the rest. Even though she is paralyzed for the duration of the fight it can still be easy to die. The best way to counteract her Captivating Song is to simply have Leliana use it. That way Marjolaine's bodyguards will also be slowed down enough for your tanks to kill them first. Then you can take out Marjolaine at your leisure.
- "Leliana so lovely to see you again, my dear"
- "Oh, you must excuse the shabby accommodations... I try to be a good host, but you see what I to work with? This country smells like wet dog. Everywhere. I cannot get the smell out. Even now it is in my hair, my clothes... ugh."
- "So business-like, your companion."
- "Dead? Nonsense. I know you, my Leliana. I know what you are capable of four, fives men... you can dispatch easily. They were sent to give you cause to come to me. And see? Here you are."
- "In truth? You have knowledge that you can use against me. For my own safety, i cannot let you be. Did you think I did not know where you where? Did you think I would not watch my Leliana? "What is she up to?" I thought. "The quiet life, the pesant clothes, hair ragged and messy like a boy... this not here". You were planning something. I told myself. So I watched... but no letters were sent. No messages. You barely spoke to anyone. Clever, Leliana, very clever. You almost had me fooled. But then you left the Chantry, so suddenly. What conclusion should I draw? You tell me."
- "Oh, is that what you think? If I were you, I would believe nothing she says. Not a one. She will use you. You look at her and you see a simple girl... a friend, trusting and warm. It is an act."
- "Oh, but you are me. You cannot escape it. No one will understand you the way I do, because we are one and the same. Do you know why you were a master manipulator, leliana? It is because you enjoyed the game : you reveled in the power it gave you. You cannot change or deny this"