From The Art and Popular Culture Encyclopedia
Gian Giacomo Caprotti da Oreno, nicknamed Salai or Il Salaino ("The Little Unclean One" i.e., the devil) was a companion of Leonardo Da Vinci for thirty years.
Records show that, after the 1476 sodomy trial, Leonardo had two long-lasting associations with young men. These were his pupils who entered his household in 1490 at the age of 10, and Count Francesco Melzi, the son of a Milan aristocrat who became apprenticed to Leonardo in 1506.
Vasari describes Salai as "a graceful and beautiful youth with fine curly hair," and his name appears (crossed out) on the back of an erotic drawing (ca. 1513) by the artist, The Incarnate Angel; rediscovered in 1991 in a German collection, it is one of the number of erotic drawings of Salai (and others?) by Leonardo once in the British Royal Collection, and is possibly a humorous take on his St. John the Baptist. The "Little Devil" lived up to his nickname: a year after his entering the household Leonardo made a list of the boy’s misdemeanours, calling him "a thief, a liar, stubborn, and a glutton." But despite Salai's thievery and general delinquency - he made off with money and valuables on at least five occasions, spent a fortune on apparel, including twenty-four pairs of shoes, and eventually died in a duel - he remained Leonardo's companion, servant, and assistant for thirty years, and at Leonardo's death he was bequeathed the Mona Lisa, a valuable piece even then, valued in Salai's own will at the equivalent of £200,000.
A group of Italian researchers, among which is Silvano Vinceti, has claimed that Salaì was the model for the Mona Lisa, noting the similarity in some of the facial features, particularly the nose and mouth, to those in which Salai is thought to have been the model. These claims have been disputed by the Louvre.