Theoden was born in 2948 TA and reigned in 2980-3019 TA. He was the 17th King of Rohan, of the former province of Gondor. He reigned at the time of the War of the Ring.
Theoden equally to Denethor, the Steward of Gondor, was a highly significant King of Men. He never fell out with his neighbors and helped Gondor's people in the best possible way. However, Rohan's power and might were restricted by the Enemy during troubled times of the War of the Ring,. Cunning Saruman secretly sent his spy, Grima the Wormtongue, who had a terrible influence on the King of Rohan via means of his speeches. That is what made the King to be on alert while dealing with strangers. Subsequently, he assigned guards to be put by the entrance to his chambers in order to disarm everyone wishing to enter Theoden's chambers.
We meet Theoden in "Two Towers" and see him as "a man so bent with age that he seemed almost a dwarf; but his white hair was long and thick and fell in great braids from beneath a thin golden circle set upon his brow. In the centre upon his forehead shone a single white diamond. His beard was laid like snow upon his knees; but his eyes still burned with a bright light, glinting as he gazed at the strangers." Theoden obtained that look through Grima's tricks and sorcery. But Gandalf broke the spells and "the black staff fell from king's hand clattering on the stones. He drew himself up, slowly, as a man that is stiff from long bending over some dull toil. Now tall and straight he stood, and his eyes were blue as he looked into the opening sky." Theoden himself realized that "dark have been my dreams of late ... I would now that you had come before, Gandalf". Theoden's transformation is highly symbolic as it reveals superiority of Good over Evil.
Theoden was a dashing horseman and warrior. Not only former strength but valor revived inside the King. He did not hide himself behind the walls of Rohan, but led his army and shared the common fate on the battlefields.
Despite his royal pride, Theoden was self-critical and capable of admitting his mistakes alongside with being a person of great wisdom. Thus, before setting off for the War, he appointed "Eomer, sister-son" to the throne. But during the War Eowyn, Eomer's sister, took the reigns of ruling.
Like any other king, Theoden liked to give feasts. Also he was generous with his companions-in-arms. Thus, Theoden gave Gandalf the best horse from his herd and the wizard's companions were given the best armor.
Theoden is a symbol for the unity of his people. He boosted his army's enthusiasm by appearing at Helm's Deep which lead to the Uruk-hai's defeat. For Saruman's henchmen, the King was a luscious prey, which they never got.
However, the King was in some doubt about the successful outcome of the battle. He prepared himself for the worst, but did not lose courage. Having freed himself from the imprisonment, the King never stopped admiring the beauty of Middle-earth and its lovely creatures: the Ents, who overthrew the stronghold of Saruman, and brave little hobbits. The King was very courteous with new acquaintances. He adored stories of unknown lands and therefore invited Merry and Pippin to his palace, with intention of listening to their stories.
To some extent, Theoden could be compared with the scales, as he constantly appears to fell under the influence of two opposite forces: Good and Evil. At first Grima and then Saruman tempted the King with sweet talks, nevertheless, the King managed to resist the temptation.
Theoden's fate had been sealed and he was doomed never to rest on his laurels in Golden Halls, and his horse was hit by a Nazgul. His last words were unbearably anguished. But before passing away he inspired all those who fought in the name of Good with his smile. Theoden managed to achieve another great objective, which was to greet a new King of Rohan on the Pelennor.
Brave warriors carried away dead Theoden from the battlefield, and then mourned their King's death. Rohan lost its King but he died like a true hero, who did not stay in gold mansions but led his people after him. If Theoden had not been with his people we would have never known the outcome of the most important battle of the War of the Ring. Theoden's greatness was in his wisdom and valor. Long live such heroes!