Legendary Side Of Things
From the legendary side of things, King Arthur was certainly someone who actually existed, albeit in a much different form than modern audiences would be familiar with. The earliest mentions of him are as a historical British king who lived during the 5th Century – he’s in a king list, he’s mentioned as winning a series of decisive battles against the Saxons, and his excellent swordsmanship is referenced once. And that’s about it.
During the Middle Ages, Arthur’s tale was romanticized and then conflated with other myths & folklore – several of his knights, such as Lancelot, were originally heroes of their own legends before being added to his – until we finally ended up with our present version that has all the Medieval knights and magic and the Round Table and the Holy Grail business. It’s entirely possible that the name of Arthur itself was part of this later conflation, as his victories against the Saxons were also credited in some sources to a general named Ambrosius Aurelianus. Those sources may be wrong, or they may have been two different leaders who were mistakenly combined by later writers (a king and his general, perhaps).