Howell's (Hoel) son and the brother of Isoud la Blanche Mains. Kehydius was with Tristram and La Blanche Mains when they blew ashore near the Isle of Servage and Tristram slew Sir Nabon le Noire. Kehydius later accompanied Tristram, Gouvernail, and Bragwaine back to Cornwall; this time they were blown ashore near Castle Perilous, North Wales, where Tristram took Kehydius into the forest with him for a few days of adventure, leaving Gouvernail with Bragwaine at the boat.
Kehydius begged for and obtained first chance to joust with the first knight they encountered. He promptly got a fall and severe wound - the knight was Lamorak, who proceeded to give Tristram himself a fall, though Tristram redeemed his honor with sword, and they ended by agreeing never to fight each other again.
When Tristram and his party finally got to Mark's court, Kehydius fell in love with La Beale Isoud and wrote love letter to her. She felt sorry for him and wrote a letter in return. Malory does not say what, exactly, was in her letter; but when Tristram found both sides of the correspondence, he misconstructed the situation, charged Isoud with falseness, and rounded on Kehydius with drawn sword. While Isoud swooned, Kehydius jumped out the window and landed in the middle of Mark's chess game, much to Mark's astonishment. Kehydius, thinking quickly, covered up by saying he had fallen asleep at the window and so tumbled out.
This incident reads like domestic comedy; the outcome of the whole affair, however, was tragic for Kehydius, who at last died of his love, as her other unrequited lover, Palomides, remarks to Tristram later at Joyous Garde:
[W]ell I wot it shall befall me as for her love as befell to the noble knight Sir Kehydius, that died for the love of La Beale Isoud.
Kehydius is a form of Kahedrin (Kahedin) used by Sir Thomas Malory.