The Romance of Tristan is actually the romance of Tristan and Yseut, his lady. If that's the case, then why isn't she in the title? Well, folks, this is a medieval romance, a genre that is usually all about the knight.
The rules of the genre say that this is supposed to be mainly Tristan's story: it starts with his birth, ends with his death and, in the moments when the lovers are separated, usually follows Tristan rather than Yseut. The feminist in you may cringe, but take heart!: The Romance of Tristan actually spends a lot more time on its heroine than most medieval romances. Check out "Protagonist(s)" in the "Characters" section for more.