Geoffrey based parts of Merlin's character on Myrddin Wyllt, a legendary madman and prophet who lived in the Caledonian Forest. Other parts of his life were based on that of the historical 5th-century Romano-British military leader Ambrosius Aurelianus (also known as Emrys Wledig). In Geoffrey's version of the tales and later embellishments Merlin is a magician and counselor for King Arthur.
According to tales first recorded in Old French in the 12th century, Yseut or Ysolt was an Irish princess betrothed to King Mark of Cornwall. After accidentally drinking a love potion, she became the lover of his nephew Tristan. Their tragic story, which was set in the Arthurian world, was popular during the Middle Ages and the name became relatively common in England at that time. It was rare by the 19th century, though some interest was generated by Richard Wagner's opera Tristan und Isolde (1865).
The Cornish form of this name, Jennifer, has become popular in the English-speaking world.
This name appears briefly in a poem in the Black Book of Carmarthen in the form Kyrridven [1] and in a poem in the Book of Taliesin in the form Kerrituen [2]. Some theories connect her to an otherwise unattested Celtic goddess of inspiration, and suppose her name is related to Welsh cerdd "poetry".