What is King Theoden's newphew's name?

King Théoden’s nephew is Éomer. In J. R. R. Tolkien’s Middle-earth, Éomer is the son of Théoden’s sister Théodwyn and the warrior Éomund, making him and Éowyn the king’s nephew and niece.

Éomer is introduced as a skilled rider and the Third Marshal of the Riddermark, the cavalry territory of Rohan. His loyalty to Rohan brings him into conflict with Gríma Wormtongue, whose influence over Théoden helps weaken the king’s judgment. Éomer is briefly imprisoned, but he later returns to defend his people against Saruman’s forces.

After Théoden dies during the Battle of the Pelennor Fields, Éomer becomes the next King of Rohan. Théodred, one of the wrong options, was Théoden’s son rather than his nephew. Gamling was a Rohan commander, while Thengel was Théoden’s father.