Éomer is of the House of Eorl and the nephew of King Théoden of Rohan. Éomer was born in T.A. 2991 as the son of Éomund and Théodwyn, youngest sister of the King. Éomer's younger sister by four years was Éowyn. He was orphaned at the age of eleven after his father was killed by Orcs, and his mother died soon after. He and his sister were taken in by their uncle. Éomer was third Marshal of the Riddermark.
During the War of the Ring, Éomer and his éored killed Saruman's orcs at the border of Fangorn Forest, allowing Peregrin Took and Meriadoc Brandybuck to escape in the chaos. On the return to Edoras, he met Aragorn and company. He allowed them to go freely and lend them two horses, on the promise that they returned to Edoras after their search was done. On return to Edoras, Éomer was imprisoned for pursuing the orcs and allowning the strangers to go free. He was let out again after the arrival of Gandalf. Before Théoden rode off to Helm's Deep, he made Éomer his official heir, as his only son Théodred had been slain before. Éomer fought in the Battle of the Hornburg and the Battle of the Pelennor Fields. There King Théoden was killed, making Éomer King of the Rohirrim. Éomer also took part in the Battle of the Morannon.
After the War of the Ring, Éomer was officially made 18th King of Rohan. He later became known as Éomer Éadig ("the Blessed"). He wedded Lothíriel of Dol Amroth, and was later succeeded by their son Elfwine.
- 'I have no child. Théodred my son is slain. I name Éomer my sister-son to be my heir. If neither of us return, then choose a new lord as you will.' - J. R. R. Tolkien, The Lord of the Rings, Book III, Chapter 6 "The King of the Golden Hall".