Éowyn is one of the main victims of Gríma Wormtongue's poisonous counsel; he convinces her that the Rohirrim are doomed to become nothing and that she will spend the rest of her life as the nursemaid to her uncle Théoden as he declines, rather than do great things as she wants. As a result, she becomes essentially suicidal, wanting to ride with Rohan's armies and die in battle. After meeting Aragorn, she develops something of an unrequited crush on him, which, as her brother Éomer later realizes, is the last straw.
Disguising herself as "Dernhelm" and taking Merry with her, she rides with the rest of the Rohirrim to break the siege of Gondor. When the Witch-king of Angmar attacks her fallen uncle, she fights him and, with Merry's help, destroys him. She is gravely injured as a result, but later cured of her physical injury by Aragorn. However, her first wish upon being cured is to be allowed to go out and die in battle, but because the armies had already left for Mordor, she stays in the Houses of Healing. There she meets Faramir, and together they help each other heal and deal with being left behind. Éowyn decides that she would rather live than die. After Sauron is defeated, she and Faramir marry.
Éowyn is often denied her heroic role, and sometimes even her courage, sometimes in favor of a Mary Sue. However, most of the time she is ignored entirely, making her a useful ally to PPC agents in need. Like Gimli, she is highly resistant to being OOC and all too happy to help destroy any threat to her world.