Short answers, pulled from the story.
Most modern historians do not consider King Arthur a historical figure. He is absent from Gildas's 6th-century account of the Battle of Badon, from Bede's early-8th-century history, and from any surviving manuscript written between 400 and 820. The historian David Dumville wrote that there is no historical evidence about Arthur and that he should be rejected from our histories.
The most widely accepted origin of the name Arthur is the Roman family name Artorius, though its own etymology is contested. The linguist Stephan Zimmer proposes a Celtic source meaning son of the bear or warrior-king. A less accepted theory traces it to Arcturus, the brightest star in the constellation Boötes near the Great Bear.
Geoffrey of Monmouth gave Arthur his first full narrative in the Historia Regum Britanniae, completed around 1138. He introduced Uther Pendragon, Merlin, the conception at Tintagel, the final battle at Camlann and the rest in Avalon. Over 200 Latin manuscripts of his work survive.
Welsh sources portray Arthur as a leader of the post-Roman Britons fighting the Anglo-Saxons in the late 5th and early 6th centuries. The Annales Cambriae dates the Battle of Badon to 516-518 and the Battle of Camlann, where Arthur and Medraut both died, to 537-539.
Lancelot was added later by the 12th-century French writer Chrétien de Troyes, who introduced him and his affair with Guinevere in Lancelot, the Knight of the Cart. Chrétien also introduced the Holy Grail, beginning the genre of Arthurian romance in which Arthur is often sidelined.
Malory's Le Morte d'Arthur was reprinted in 1816 for the first time since 1634, helped by medievalism, Romanticism and the Gothic Revival. Alfred Tennyson became central to the revival, and his Idylls of the King, first published in 1859, sold 10,000 copies in its first week.