Short answers, pulled from the story.
The earliest datable mention of King Arthur comes from the 9th-century Latin historical compilation known as the Historia Brittonum. This text lists twelve battles he fought, culminating in the Battle of Badon where he is said to have single-handedly killed 960 men.
The most widely accepted etymology derives the name King Arthur from the Roman nomen gentile Artorius, a family name of obscure and contested origin. Linguist Stephan Zimmer suggests that Artorius possibly had a Celtic origin, being a Latinization of a hypothetical name Artorījos, in turn derived from an older patronym Arto-rīg-ios, meaning son of the bear or warrior-king.
Geoffrey of Monmouth's Historia Regum Britanniae, completed in the 1130s, transformed King Arthur from a folk hero into a king of all Britain. He incorporated Arthur's father Uther Pendragon, his magician advisor Merlin, and the story of Arthur's conception, in which Uther, disguised as his enemy Gorlois by Merlin's magic, sleeps with Gorlois's wife Igerna at Tintagel.
During the 12th and 13th centuries, the role of King Arthur in continental romance shifted dramatically from a great and ferocious warrior to the roi fainéant, the do-nothing king. His character became a wise, dignified, even-tempered, somewhat bland, and occasionally feeble monarch whose inactivity and acquiescence constituted a central flaw in his otherwise ideal society.
In 1191 the alleged tomb of King Arthur was identified in an orchestrated discovery at Glastonbury Abbey. This event occurred decades after the first mention of his return by William of Malmesbury in 1125 and was likely motivated by the Abbey wanting to stand out with an illustrious tomb or the Plantagenet regime to put an end to a legendary rival figure.
In the early 19th century medievalism, Romanticism and the Gothic Revival reawakened interest in King Arthur and the medieval romances, shaping a new code of ethics for 19th-century gentlemen around the chivalric ideals embodied in the Arthur of romance. This renewed interest first made itself felt in 1816, when Malory's Le Morte d'Arthur was reprinted for the first time since 1634.