Ethelred the Pious
He's the title character, and yet he dies two-and-a-half years into the timeline. Poor king!
Ethelred of Wessex, remembered forever as Ethelred the Pious, was the fourth son of King Ethelwulf and the older brother of King Alfred. He was the last King of Wessex to have control over the kingdom.
Ethelred was born c. 840 and grew up to be a very religious man and very dedicated to his God and country. succeeded his older brother to the throne at the age of 25, during the height of the Viking invasions of the Ninth Century. Ethered suffered a major defeat to a large Danish army on January 4, 871, at Reading, although the Saxons did manage to inflict heavy casualties on their enemies. Four days later on January 8, the armies faced each other once again, at the Battle of Ashdown. There, Ethelred tarried for too long in a nearby church, receiving the Sacrament and praying for victory. His brother Alfred respected his wish to hold the Saxon army until their king finished-- which allowed the Danes to gain the upper hand and rout the Saxons.
For the next two years, Ethelred waged a brave but unsuccessful defensive campaign. In March he rallied his remaining troops and led a counterattack on the Danes. This was not a simple "re-match" of Ashdown (as it was in OTL) but a desperate gamble against a far superior force. Knowing that another rout would mean a swift end to Wessex, Ethelred withdrew in order to save a remnant of his forces. Unlike OTL, he survived, but his attack failed.
In the summer the Danes took the key West Saxon stronghold of Wareham and held it for the winter. The following spring, Ethelred went on the offensive again, driving the Danish army into the Somerset Swamps and winning a great victory there. However, for the next year Ethelred led his army in one long, ignominious eastward retreat. Driven into Kent, Ethelred was literally backed into the island's corner. He met his end at the Battle of Headcorn in Kent, after which Alfred assumed the throne but was unable to reverse the Viking conquest.
In the Alfredssaga Ethelred is portrayed as a noble yet naive warrior for Christ, riding boldly into the Danish lines and cutting down the Heathen with his sword. He is betrayed by cowardly underlings, clergy in league with the Vikings. They lead him astray by taking advantage of his simple faith, convincing him that various ill-advised moves are actually the will of God.