The answer to this question depends upon if one actually believes that King Arthur existed or not. Historians have argued in favor of both sides. Some have found papers which denote his lineage (as the son of Uther Pendragon and Igraine of Cornwall), while others have dismissed him as the creation of many imaginative tales.
Those who dismiss Arthur, as a figment of the imagination, state him to be the one character who was created as the result of the actions and stories regarding many true warriors (those who followed the code of chivalry and of the knight). Others state that Arthur was, in fact, a real person (just not the king he has come to be known as).
Essentially, King Arthur could be a real person who ruled his own kingdom. King Arthur could also simply be a man created to illuminate and illustrate the ideals of a culture. In the end, no definitive answer has ever been found.
In reality, no one really knows. However, one popular theory is that King Arthur is based off of a famous Roman general. Sadly, I forgot his name, but there was actually a show about this. If you look it up, I'm sure that you will find it! It is actually quite interesting.