So as a preface, I haven't actually watched the Last Kingdom so I won't concentrate on a comparison between the media and history. However, sources from the period are quite scarce and vague compared to the detail we get from other periods both before and after. Works of fiction actually have considerably more room to play around in Alfred's era since there is a lot of things we simply lack any details on. That being said, Alfred is a prominent figure during this period precisely because he is one of the first people we have substantial written sources on which are the backbone for forming any kind of historical narrative of the period.

The main source we have on Alfred is a biography written by a Welsh Bishop, Asser of Sherbourne while Alfred was still alive. Asser's Life of Alfred details Alfred's life and achievements and the work was directly patronized by Alfred. So our most detailed source directly detailing Alfred's life is, for a lack of a better work, royal propaganda. This does not mean the source is useless or outright lying; though everything that is written needs to be taken with a mammoth-sized grain of salt. Our other source is the Anglo-Saxon Chronicle which many historians like James Campbell have argued is "prima facie" a piece of propaganda. So this leaves us in a bit of a bind if we want to know what Alfred was "really" like since all our sources are created with the intent of display a certain image. This is not unusual in history, in fact, it is the norm; who wants to leave behind a chronicle of their shortcomings? What makes interrogation of our sources difficult is that we lack other sources to effectively compare them with. Unlike later periods and even the preceding classical period before it, there aren't any private letters or a large body of text that we can compare. Despite this difficulty, both our main textual sources do give us sufficient evidence that Alfred was at the very least an able military and political leader. However, Alfred's reputation is quite complex since it evolved over time and the vagueness of sources has allowed for multiple interpretations.

Alfred was first given the title "the Great" during the Elizabethian era, more than 500 years after his death. Much of Alfred's reputation prior to 20th Century was essentially manufactured for political gain. Alfred became, as Barbara Yorke has aptly put, "The Most Perfect Man in History" due to the constant revision of Alfred's image. Alfred was seen or conceived to be the founder of England and as such there was a great deal of political prestige and capital to be gained by having the current monarch's reign draw lineage or compared to his. His next large boost in popularity came with the translation of Asser's Life of Alfred to modern English by Sir John Spelman in the 17th Century which made him more visible. It is in the 18th Century that Alfred's reputation started to reach close to mythic heights. The Alfredian craze began with the tributes by backers of Fredrick Prince of Wales against Robert Walpole. They chose to style Frederick as the "patriot king" who had saved his country from tyranny and were attempting to link him with Fredrick so that he may do the same upon his coronation. The narrative surrounding him was reinvented which led to him being hailed as the foundation of a proto-democratic government that Parliamentary Britain drew linage form and that was briefly suppressed by Norman tyranny after their conquest in 1066. It was during this period that many tributes to Alfred started to appear, most famous of which was the 1740 Opera Alfred, a Masque where the song Rule Britannia was first played. Other tributes include William Kents quite absurd sounding Temple of British Worthies which had a bust of King Alfred praising him as the founder of the English Constitution and the Protector of Liberty. By the Victorian period in the 19th Century, Alfred had become the idealized person espousing Victorian values. He was remembered in Charles Dickens's a Child's History of England as a perfect ruler. Although this was meant to be for juvenile consumption, it began to popularize the image of Alfred further. Due to the craze concerning all things German during the Victorian era, Alfred was at this point hailed as the founder of the Royal Navy and founder of the British Empire which was a vast exaggeration, to say the least. This was followed up by many commemorations of the millenary of his birth and death with statues at Wantage and Winchester. Finally, he had the Drake-Class Armoured Cruiser HMS King Alfred named after him. So suffice to say, he was considered "Great" long after he was dead, and his reputation had ballooned out of proportion to any of his achievements.

Awesome. Thank you guys a lot!