Iseult

I heard the Round Table story as Tristram and Iseult, not Tristan and Isolde, and that there were two Iseults, one of them his jealous wife. Because this was the first version I heard, I am rather fond of this name.

Also, Iseult was the daughter of King Anguish of Ireland who was the intended bride of King Mark of Cornwall, but as a result of drinking a love-potion, hopelessly enamoured of Tristan. When she heard of Tristan's death, she died of a broken heart. Her name is not Irish, but derived from Ancient British Adsiltia (she who is gazed on). Attempts to associate her with Chapelizod, Dublin, are due to a false derivation of that place name.

---

Or, at least, so says some websites. :)

In some versions of the story of Isolde and Tristan, Iseult, rather than being an alternative spelling for Isolde, is a character in her own right. She is a noblewoman who bears a striking resemblance to Isolde, so Tristan marries her to try and forget about Isolde. But on his deathbed he calls for Isolde, and Iseult, jealous that her husband loves another better than her, tells him that Isolde is not coming and he dies instantly from grief. Isolde, having heard she missed Tristan's last moments, dies of a broken heart soon after.