Guinevere is an adaptation of the Welsh name, “white shadow” — later, the name of a Peter Gabriel song.
The Arthurian legend has universal appeal. It notes the noblest of human endeavor, betrayal, forgiveness and hope for restitution. The tale has subsequently attracted a number of additional characters who only add depth to the basic appeal. It somewhat mimics ancient Greek myths such as Odysseus.
Geoffrey of Monmouth knew a good story when he found one. He could have produced great movies.