Leonardo provided housing and support for a young boy variously described as an adoptive son, protégé, companion, or servant. But Gian Giacomo Caprotti da Oreno wasn't nicknamed Salai, meaning "Little Satan", for nothing.

Soon after Leonardo had taken the 10-year-old rascal into his home he stole the money put aside to buy him two new shirts, a pair of shoes and a doublet. In Leonardo's own words the child was, "thievish, lying, obstinate, greedy." It seemed he never improved. Leonardo recorded various disasters from the child including stealing, bad manners and gluttony. In his first year with Leonardo, Salai was given a cloak, six shirts, three doublets and at least twenty-four pairs of shoes.  He continued to steal at every opportunity and when in public Leonardo kept him close by so he could watch his activities.

He saw Salai attempt to steal money from the purse of a footman who had undressed to try on a jousting costume Leonardo had made. Not long after this he stole a fine piece of Turkish hide which Leonardo was going to have made into boots. After selling the hide to a cobbler Salai spent the money on anise candy.

Salai is said to have been a handsome, almost pretty boy with long golden ringlets. It is thought to be he who provided the model for the youth in Leonardo's Portrait of an Old Man and a Youth. Salai showed some small talent for painting, but did not become a painter of any note. Leonardo taught him much and often touched up his work and the suggestion has been made that Salai also pandered to Leonardo's homosexual instincts.

Despite the problems, and the ever-continuing stealing, the association between the two was close and he stayed with Leonardo until the artist's dying day. In total, Leonardo supported him for twenty-five years and left Salai half of his vineyard in his will.

Salai appears to have died in 1523, probably of a gunshot wound.