King Arthur (キング·アーサー, Kingu Āsā; full name "Arthur Pendragon") was the legendary warrior knight and king of Britain in the 12th century as well as one of the Knights of the Round Table who was formerly the well-known Meister of the most powerful weapon in the world, Excalibur. While mentioned in the manga, he only appears in the anime.
Arthur is seen wearing armor resembling that of a Greek or Roman origin with a golden helmet bearing a Mohican style plume and silver armor adorned by red cloth flaps. Arthur has black hair, a thick black beard and a small mustache. In one instance he is shown wearing Excalibur in a hilt on his sword belt.
Arthur is portrayed as a good and noble king, albeit one who sometimes doubted himself. His creation of the Round Table portrayed him as someone who trusted his knights as equals and valued their friendship.
Little is known about the individual. It was known, however, that he was formerly the wielder of the Holy Sword.
This section contains an instance from an anime not present in the manga and/or a deviation
In the anime, more of Arthur is expanded on.
Born in Medieval Britain, Arthur grew up to become king of the land, though it was divided among the many tribes. One day, he was alerted to the existence of The Holy Sword and set out to obtain it. Passing Excalibur's test, Arthur became the wielder of Excalibur, and proceeded to unite all the tribes in Britain, ruling over "all under the sun", as Excalibur put it.
Arthur then gathered a troupe of his most loyal knights around him and together they formed The Knights of the Round Table. In addition to fighting evil, Arthur and his knights went on various quests and fought crusades against the Romans, and possibly the Witches. Excalibur, who was as much a personal friend of the King as his Weapon, was instrumental in many, though not quite all, of the victories.
After many years, Arthur died, and Excalibur returned to his dwelling.
↑Soul Eater Manga: Chapter 6 - While "King Aruthur" is simple mention as that, his full real name is Arthur Pendragon.