Cador, ruler of Cornwall (variously described as a king or a duke) is a character in Arthurian legend. The Welsh version of his name, Cadwyr, can be translated literally as "battle-man."
There was a historical king of Dumnonia (present day Devon) named "Cado" (Cadorius) in Latin), who was the son of a King named "Gerren." Little else is known about him, but this man seems to have been incorporated into the Arthurian legends as a friend and comrade of King Arthur. Various places named "Cadbury" may have been formerly hillforts belonging to Cado and thus named "Cado's fort."
In Geoffrey of Monmouth's History of the Kings of Britain and various post-Galfridian sources that follow Geoffrey's version, Arthur's queen Guinevere is raised in Cornwall as Cador's ward, and Cador's son Constantine inherits Arthur's kingdom upon his death. Cador himself fights at Arthur's side against the Saxons, defeating the Saxon warlords Baldulf and Cheldric.
In The Dream of Rhonabwy, a medieval romance associated with the Mabinogion Cador is "Cadwr, Earl of Cornwall," the man given the honor of personally arming King Arthur on the day of Arthur's great victory over the Saxons at the Battle of Badon Hill, and in the Life of Saint Carantoc Cador and Arthur are even said to co-rule a kingdom together.
In a marginal note to a translation of the Brut Tysilio, the anonymous translator asserts that Cador was son of Gorlois, presumably by Igraine, which would make him Arthur's maternal half-brother and perhaps explain their rather chummy relationship. But this is hard to square with the account in Layamon's Brut which claims that Cador takes charge of Uther Pendragon's army when it is attacked by Gorlois while Uther is secretly lying with Igraine in Tintagel Castle.