Little else is known about Théodred, except that he loved Éomer like a brother, and the only person he loved more than Éomer was his father, Théoden.
Théodred held the title of Second Marshal of the Mark, though he was slain in the First Battle of the Fords of Isen defending the islet in the middle of the ford. With no siblings, this meant making his cousin, Éomer, heir to the throne of Rohan.
Théodred's name is probably taken from the Anglo-Saxon words "þeod" ("folk", "people", "nation") and "ræd" ("counsel").
Portrayal in the Movies
In The Lord of the Rings: The Two Towers, the death of Théodred is a major event early in the movie, whereas, in the book, it has all happened by the time we come to Rohan.