I have a deep and abiding love for the Arthurian legends, leading back to when I was probably less than four years old. I also have studied medieval and "Late Antiquity" history a fair amount (my degree is in Medieval History). By combining the two, along with a long love of literature in general I can say this about the historical King Arthur: if he existed as a single figure, which I very much doubt, we would not recognize him as such. Whether you go the dux bellorum line, the Last of the Romans line, the Celtic Revival line, the Sarmatian Import line, or anything else, there is very little we can say about post-Roman Britain with any degree of certainty other than the fact that the Angles, Saxons, and Jutes (along with their various allies) poured into Britain in great numbers; between the Retreat of the Legions in 415 and the establishment of the Heptarchy two centuries later, there is a lot of mist, a lot of conjecture, and a fair amount of intriguing, yet inconclusive, archaeological evidence.
This is one of the many reasons that the Arthurian legends continue to exist in so many different forms -- you can choose your emphasis, drawing on multiple, semi-connected strands of legend, history, and pure mystery. Nennius doesn't give us much to go on. The Mabinogian was written down so late that it is almost of necessity corrupted.
So who was the "real" Arthur? My thought is that "he" was probably not even one guy, but a series of people whose stories got woven together through the intervening centuries. To his tale was added various tales of Myrddin/Merlin, Taliensin, Merdaut/Mordred, Gwalchmai/Gavin/Gawain, the Grail/Cauldron, Avalon, Joseph of Arimethea, Tristram/Drustan and Isolde/Issuelt/Yseld, and all the rest -- think of "Arthur" as the gravity well that drags all the other nearby tales into a erratic orbit.
Don't look to closely for reality -- you will always be disappointed
OTOH, there have been some fantastic writers over the years: Geoffrey of Monmouth, Chretien de Troyes, Thomas Mallory, Alfred, Lord Tennyson, T.H. White, Vera Chapman, Phyllis Ann Carr, Thomas Berger, to give only the mildest spot check.
In other words, when it comes to King Arthur, there is little truth, much speculation, and some wildly divergent literature, from the truly amazing to the truly awful.
Personally, I am putting together my "Arthurian Limberger" collection, including pictures of the Excalibur Hotel in Las Vegas, a copy of the Planet of the Apes comic book where the characters "inadverently" re-enact Arthurian tropes, and some amazingly cheesy movies!
