Sir Gareth was a Knight of the Round Table in Arthurian Legend. He was the son of Lot and of Morgause, King Arthur's sister, thus making him Arthur's nephew, as well as brother to Gawain, Gaheris, Agravaine, and half brother of Mordred. He is the subject of Book VII in Sir Thomas Malory's Le Morte d'Arthur, which tells how he became a knight.
According to Malory's tale, Gareth came to Camelot in disguise as a kitchen boy and was set to work by Kay, who always gave him difficult work, teased him as being a lowly kitchen boy, and nicknamed him "Beaumains" or "Good Hands" (alternatively "White Hands", or "Beautiful Hands"). The meaning of this was that Gareth's hands were white or soft, showing he was not used to hard work. In the tale Gareth goes to the aid of Lynette (sometimes Lyonet or Lyonette), to save her sister Lyonesse (or Lyonorr), from the Red Knight of the Red Lands. He is accompanied by the dwarf Melot who knew his true identity.
However, Lynette thought Gareth to be a mere kitchen boy and constantly derides him. On the way he defeats the impressive Sir Perarde, Black Knight, and takes his armor and horse. He then meets Sir Pertolope, the Green Knight, who mistakes him for his brother, the Black Knight. Lynette tells the Green Knight that he is Beaumains, a kitchen boy, and not his brother the Black Knight and begs him to rid her of him. However, Gareth overcomes the Green Knight, and spares his life in return for the knight's swearing to serve him. He then sets out and in much the same fashion defeats Sir Perymones, the Puce Knight (sometimes the Red Knight, not to be confused with the one of the Red Lands), and Sir Persaunte (Persant of Inde), the Indigo Knight, both of whom also swear to serve him. Lynette finally sees that Gareth's calm acceptance of her abuse is very knightly, and that he must be a very good knight indeed and no mere kitchen boy.
He finally arrives at Lyonesse's castle, where she is besieged by the Sir Ironside, Red Knight of the Red Lands. He fights him, which takes all day, and finally prevails, although the Red Knight had the strength of seven men. He originally intends to kill him, as the Red Knight slaughtered all the other knights who came to save the lady Lyonesse. However, the Red Knight explains that he was doing so because the lady he loved made him swear to kill Lancelot, and the only way to get his attention was to kill the knights. And so Gareth spares him, making him swear to serve him and also go to Arthur's castle and apologize to Lancelot. Afterwards, and despite some difficulties, Gareth marries Lyonesse. Gareth also kills King Datis of Tuscany. Unfortunately, he is later killed accidentally by Lancelot during a rescue of Guinevere.
The legend has been reinterpreted by many writers and poets, the most renowned being Alfred Lord Tennyson in Idylls of the King, wherein the colored knights are replaced by knights associated with various times of day. The final knight is known as Night or Death, and he is the most feared of the three, though ultimately the weakest. In this version, it is not clear whether Gareth marries Lynette or Lyonesse, though Tennyson writes that Gareth claimed to have married Lynette when recounting his own life. Theodore Goodridge Roberts authored the short story "For To Achieve Your Adventure", in which Lynette knows she is sending Gareth into an ambush, and her derision is an attempt to make him give up for his own protection. Vera Chapman's novel The King's Damosel gives a complete version of Lynette's life.