This relatively obscure figure is first mentioned in Culhwch and Olwen, where her name is among those 200 men, women, dogs, and horses invoked by the hero Culhwch to punctuate his request that King Arthur help him find his love Olwen. Both of the Triads that mention Gwenhwyfach refer to the enmity between her and her sister that led to the Battle of Camlann. Triad 53 lists as one of the "Three Harmful Blows of the Island of Britain" the slap that Gwenhwyvach gave her sister that caused the Strife of Camlann. Identifying Camlann as one of Britain's "Three Futile Battles", Triad 84 mentions it was started because of a dispute between the sisters. Some have suggested that "Gwenhwyfach" in Triad 53 is a mistake for "Medrawd", since Triad 54 describes Medrawd raiding Arthur's court and throwing Gwenhwyfar to the ground and beating her; this interpretation does not explain Triad 84, however. Rachel Bromwich notes, citing the spelling found in Culhwch and Olwen and Triad 84, that Gwenhwyach may in fact be the original spelling of the name. Melville Richards and Bromwich previously suggested that the alternate spelling of her name in medieval Welsh sources, Gwenhwywach, could have been understood as Gwenhwy-fach, or "Gwenhwy the Lesser", a back-formation based on a false etymology of her sister's name as Gwenhwy-, meaning "Gwenhwy the Great". It is possible that Gwenhwyfach was once thought of as a darker aspect of Gwenhwyfar.
The Lancelot-Grail cycle introduced a possibly related character known as "the False Guinevere" or "Guinevere the False". Named Genievre, she is the real Guinevere's identical but evil half-sister by a different mother. Also known as the Lady of Camelide, she bewitches Arthur and turns him against the real Guinevere.
Some modern writers associate Gwenhwyfach with Mordred, presumably due to her association with Camlann; she appears as the traitor's wife in Thomas Love Peacock's novel The Misfortunes of Elphin, for example. In Bernard Cornwell's Enemy of God, she is Guinevere's delusional and dim-witted fat sister who aids Arthur in his supposed "rescue" of Guinevere from Lancelot's castle, and later goes completely insane while living alone waiting for Lancelot to arrive.
