The dashing Alfonso X
via Wikimedia Commons
Often characterized as his most romantic relationship, the affair started when he was just a teenager. She was about 16 years older. It continued while he waited for his child-bride Violante of Aragon to grow up. Born into an extremely aristocratic family, Mayor seems to have suffered little ill-will because of the affair. Instead, she was showered with lands. She and Alfonso even founded the Monastery of Santa Clara de Alcocer together. Decades later, after her death, he even erected a beautiful sepulchre in here memory. It survived until the Spanish Civil War in 1936.
The couple's only child together, Beatrice, was one of Alfonso's favorite children--he had 11 with his wife and a smattering of other bastards. In fact, Beatrice made a better marriage than any of her legitimate sisters: she married the King of Portugal.
Lest you think, that Alfonso and Mayor's relationship kept him completely satisfied, however, it is worth mentioning that Beatrice was born the same year as her illegitimate brother Alfonso Fernandéz de Castilla.
More about Mayor (both in Spanish):
Amante del Ray Sabio, Doña Mayor Guillén on Mujeres en la Historia
Doña Mayor Guillén y Alfonso X el Sabio, el amante on La Aventura de la Historia