In The Lord of the Rings: The Two Towers, Théoden’s niece is Éowyn.
Éowyn is a noblewoman of Rohan and the daughter of Éomund and Théodwyn. Because Théodwyn was Théoden’s sister, Éowyn is Théoden’s niece. She lives in Edoras and helps care for the king while Rohan faces pressure from Saruman’s forces and internal corruption.
The films portray Éowyn as a skilled fighter who wants to defend her people rather than remain confined to a traditional courtly role. She develops feelings for Aragorn, although Aragorn ultimately chooses Arwen. Éowyn later forms a relationship with Faramir after the Battle of the Pelennor Fields.
A notable distinction between the film and book versions concerns her battlefield identity. In both versions, Éowyn disguises herself as a soldier named Dernhelm and confronts the Witch-king of Angmar. Her declaration that she is no man fulfills the prophecy that the Witch-king would not be slain by a man.