Wikipedia
Gaheris (Old French: Gaheriet or Gaheriez) is a character in the Arthurian legend, a nephew of King Arthur and a Knight of the Round Table, the third son of Arthur's sister or half-sister Morgause and her husband Lot, King of Orkney and Lothian. He is the younger brother of Gawain and Agravaine, and the older brother of Gareth and half-brother of Mordred.
Gaheris is mentioned in Chrétien de Troyes's Perceval, the Story of the Grail. Several of his adventures are narrated in the Lancelot-Grail cycle. In the prose Lancelot, he is described as valiant, agile, handsome, reticent in speech, prone to excess when angered, and possessing a right arm longer than the left. His death at the hands of Lancelot (during Lancelot's rescue of the queen from being burned at the stake) is related in the Death of Arthur, the final volume of the cycle. The Lancelot and the Death of Arthur sections of the Lancelot-Grail cycle differ in their characterization of Gaheris. In the Lancelot, his younger brother Guerrehet/Gareth is Gawain's most cherished brother; in the Death of Arthur, Gaheris is represented as the most cherished, and his death anguishes Gawain profoundly.
The character of Gaheris appears in the Post-Vulgate Cycle and Thomas Malory's Le Morte d'Arthur, as well as in other media related to the Arthurian legend.