Guinevere, or Gwenhwyfar in Welsh, was King Arthur’s wife.

That’s pretty much all that we know about her conclusively (bearing in mind that we can hardly be conclusive about King Arthur’s existence, either–see my posts here: https://sarahwoodbury.com/all-about-king-arthur/).

She is first named in the Welsh story of Culhwch and Olwen, a tale about a hero connected with Arthur and his warriors.  We have two manuscripts: a complete version in the Red Book of Hergest, ca. 1400, and a fragmented version in The White Book of Rhydderch, ca. 1325. It is the longest of the surviving Welsh prose tale and likely existed before the 11th century, making it the earliest Arthurian tale.  http://en.wikipedia.org/wiki/Culhwch_and_Olwen

In it, Arthur says:   “Since thou wilt not remain here, chieftain, thou shalt receive the boon whatsoever thy tongue may name, as far as the wind dries, and the rain moistens, and the sun revolves, and the sea encircles, and the earth extends; save only my ship; and my mantle; and Caledvwlch, my sword; and Rhongomyant, my lance; and Wynebgwrthucher, my shield; and Carnwenhau, my dagger; and Gwenhwyvar, my wife. By the truth of Heaven, thou shalt have it cheerfully, name what thou wilt.”  She has two attendants, Elidyr Gyvarwydd and Yskyrdav, the Yscudydd, as well as a sister, Gwennhwyach. http://www.maryjones.us/ctexts/culhwch.html

Her existence is also recorded in the Welsh triads from the Red Book of Hergest. She is one of THREE Gwenhwyfar’s to whom Arthur was married:  “Gwennhwyfar daughter of Cywryd Gwent, and Gwenhwyfar daughter of Gwythyr son of Greidiawl, and Gwenhwyfar daughter of Gogfran the Giant.”

No wonder sifting out what might be real about King Arthur from what is improbable is so difficult.  In another passage, the first Gwenhwyfar is mentioned in ‘Three harmful blows to the Island of Britain’: 
“The first of them Matholwch the Irishman struck upon Branwen daughter of Llyr; The second Gwenhwyfach struck upon Gwenhwyfar: and for that cause there took place afterwards the Action of the Battle of Camlan; And the third Golydan the Poet struck upon Cadwaladr the Blessed.”

Then come the “Three unrestrained ravagings”:  “The first of them when Medrawd came to Arthur’s Court at Celliwig in Cornwall; he left neither food nor drink in the court that he did not consume. And he dragged Gwenhwyfar from her royal chair, and then he struck a blow upon her; the second Unrestrained Ravaging when Arthur came to Medrawd’s court. He left neither food nor drink in the court;  And the third Unrestrained Ravaging when Aeddan the Wily came to the court of Rhydderch the Generous at Alclud [Dumbarton]; he left neither food nor drink nor beast alive.”

That’s it.

From this point, like the rest of the King Arthur story, the person of Guinevere evolved to the story that is common knowledge today–that of an unfaithful wife who betrays Arthur with his best friend and is an accomplice in Modred’s treachery against Arthur. For a complete progression of Gwenhwyfar to the Guinevere who brings down Camelot, see:  http://www.arthurian-legend.com/more-about/more-about-arthur-9.php