Malory calls Sir Pelleas one of the six knights who could defeat Gawaine. When Gawaine first met him, during the adventure of the damsels “Printemps, Été, and Automne“, Pelleas was not yet a member of Arthur’s court. Gawaine found him diligently pursuing his love for Ettard. At a three-day tournament in Ettard’s part of the country, Pelleas had proved the best of 500 knights, winning the prize, a circlet of gold, and presenting it to Ettard as the fairest lady there (although in fact there were some fairer ones present).
And so he chose her for his sovereign lady, and never to love other but her, but she was so proud that she had scorn of him, and said that she would never love him though he would die for her. ... And so this knight promised the Lady Ettard to follow her into this country, and never to leave her till she loved him.
He lodged by a priory, and every week she sent knights to fight him. He would defeat them all, then allow them to take him prisoner so that, when they brought him in disgrace before their lady, he would get a sight of her.
And always she [did] him great despite, for sometimes she [made] her knights to tie him to his horse's tail, and some bind him under the horse's belly; thus in the most shamefullest ways
that she could devise (apparently desperate, poor lady, to be rid of him) was he brought to her. Gawaine promised Pelleas to act as a go-between:
I will have your horse and your armour, and so will I ride unto the castle and tell her that I have slain you, and so shall I come within her to cause her to cherish me, and then shall I do my true part that ye shall not fail to have the love for her.
Gawaine managed all this except the last part – he stopped before he got around to pleading Pelleas’ case. The month was May, so they went outside to take their pleasure in a pavilion. Here Pelleas found Gawaine and Ettard sleeping together, and went well-nigh mad with grief. He considered killing them, but ended by laying his naked sword “overthwart both their throats”. Then he rode back to his own pavilions and announced to his knights and squires that he was going to bed and would never get up until he was dead, and when he was dead they were to take his heart between two silver dishes to Ettard.
Apparently he changed his mind about dying in bed, because Nimue met him wandering in the woods on foot. She caused an enchantment whereby Ettard fell madly in love with Pelleas, while all his love for her turned into hate. Then Nimue got Pelleas’ love for herself, by enchantment or perhaps simple seduction, and married him, while Ettard died of love.
When Pentecost came, Nimue brought Pelleas to Arthur’s court, where he took the prize at jousting and was elected to the Round Table, at the same time as Sir Marhaus. Nimue did not keep Pelleas completely sequestered with her in her Lake; he appears fighting in at least one tournament, riding a-Maying with Guenevere, and trying to heal Sir Urre. But Nimue “saved him that he was never slain”. Probably when the Round Table was destroyed by the feud between Lancelot and Gawaine and by Mordred’s rebellion, Nimue simply took Pelleas permantely into the Lake, not letting him fight at Salisbury.
One of Caxton’s chapter headings calls Pelleas “King”. I suspect confusion with King Pelles/Pellam. Malory tells us, immediately after describing Pelleas’ election to the Round Table, that he “was one of the four that achieved the Sangreal”, but we never hear any more of this; again I suspect confusion with Pelles, or possibly Percivale. Being married to Nimue, Pelleas was hardly a virgin or, like Bors, a celibate; nor was Nimue one to have taken tamely the stricture against ladies accompanying their knights in that Quest!