Éomer, whose father Éomund was the Marshal of the Eastfold of Rohan, lost his parents early in his life, and was taken into the personal care of King Theoden along with his sister Éowyn. He and the thirteen years older son of the King, Théodred, soon became good friends, even though Gríma Wormtongue, counselor of Theoden tried to convince the King that Éomer was only after the throne himself and would be a dangerous imposter to his son. These intrigues of Wormtongue made life for Éomer and his sister harder than it would have otherwise already been.

So, when Éomer became Third Marshal of the Riddermark already in 3017, he most often took his leave to hunt down the gathering enemies and orcs with his fellow soldiers far away from Edoras, where the King was sitting and being made to believe those very orcs were his allies. With the death of Théodred at the fords of the Isen, Éomer practically became the highest military leader of the Rohirrim, and the military head of command.

By the time members of the Fellowship of the Ring entered Rohan, Éomer's fate became intertwined with theirs: the sortie of Uruk-Hai from Isengard under the lead of Úgluk was surrounded by Éomer and his men, and wiped out entirely, giving the captives Merry and Pippin the chance to escape.

After that, they came across the three hunters Aragorn, Gimli and Legolas, who were pursuing the orcs as well, and like Aragorn seemed to perceive the valour of the towering and flaxen young Marshal, so Éomer proved his sense when he realized the noble honesty of Aragorn and the importance of his errand, when he aided him and gave them three spare horses from his éored.

For that, he was imprisoned back in Edoras at the command of Wormtongue, for disobeying the orders of the King. But when Gandalf the White came to Meduseld shortly after, and healed the King from the shadow that had passed over him, Éomer was freed, and Theoden acknowledged his actual loyalty to the King and to Rohan, and rode to defend Helm's Deep, where Éomer had the actual command.

Éomer drew his sword Gúthwine ('friend in battle') together with Aragorn both there in the victorious charge beyond the Deeping Wall, and later in the daring charge of his first éored into the Pelennor to turn the tide.

With his last words, the fallen Theoden named Éomer his successor there, and he started a new third line of Kings of the Mark.

After the downfall of Sauron, Éomer married Lothíriel, daughter of Imrahil of Dol Amroth, who, like King Elessar, was one with whom he had went throught the hardships of the War of the Ring and so had become friends. Though the reign of Éomer was peaceful and prosperous for Rohan, these three often went to war even beyond Rhûn and Harad to purge the last remnants of the armies of the Dark Lord, and bring peace to those wild countries.

His last days of life Éomer spent with his old friend from the Shire, Meriadoc Brandybuck, who was called Holdwine among the Rohirrim. When he died at the respectable age of 93, in the 63rd year of the Fourth Age, and started a new line of grave mounds in Edoras, Éomer, then known as Eadig, 'the happy', was succeeded by his son Elfwine, who was able to continue his father's good reign and was soon called the Fair.

 

Info from: www.barrowdowns.com