In the Tristan tradition, Kahedin belongs to the household of Hoel in Brittany. His sister is Isolde of the White Hands, the woman Tristan marries after separation from the Irish Isolde. Friendship and family obligation therefore become entangled. Kahedin can welcome Tristan as a companion while also learning that his sister's marriage does not have the intimacy expected of it. The shared name of the two Isoldes matters to the story, but it does not turn the women into substitutes for one another or give the Breton household a simple resolution.

Kahedin's most important service in Thomas's ending is a journey rather than a battle. Tristan has suffered a poisoned wound and believes that only the Irish Isolde can heal him. He entrusts Kahedin with a ring that will identify him as a genuine messenger, along with instructions for the return voyage. The ship must display a white sail if Isolde comes and a black one if she does not. Kahedin reaches the queen and persuades her to return. He fulfils his mission, but the wounded man's survival depends on more than the messenger's loyalty.

The voyage encounters delay, and Isolde of the White Hands has learned the meaning of the signal. When the vessel approaches, she falsely tells Tristan that its sail is black. He dies before the Irish queen reaches him, and she then dies beside him. Kahedin should not be blamed for returning under the wrong colour: the deception concerns what his sister reports, not a failure to raise the agreed white sail. In this branch he is the companion who tries to save his friend; he does not first die in a separate love affair and then return as the messenger.

Other branches make his fate different. The supplied Eilhart tradition links him with Ganoje, wife of Nampetenis, and a fatal encounter during that affair. In the prose tradition, represented in Malory by Kehydius, his own love for the Irish queen creates trouble between the companions. Malory explicitly anticipates his death for that love. These endings must remain alternatives to the completed healing voyage. A character's importance can persist across retellings even when a poet or compiler changes the relationship that brings him to grief.

The wider Arthurian name tradition requires equal care. Chrétien's Perceval names a Kahedin who undertakes the adventure of the Dolorous Mount, while the Middle English Prose Merlin includes Kehedin among the young knights and calls him the nephew of Kay Destranx. Accounts of Kahedins the Small or the Fair add other kinship, campaign and captivity material. These references are not sufficient by themselves to prove that every named knight has the Breton prince's parents and history. The forms Kahedin and Kaherdin can be retained together while the separate Vulgate settings remain explicitly distinguished from Tristan's brother-in-law.