Who was michelangelo's mentor?

Gian Giacomo de' Caprotti, also called Salai, was the most famous.

Also Francesco Melzi.

When Leonardo died, he left his all his paintings that were in his possession (including The Mona Lisa) to Salai and all of his precious notebooks to Melzi.

Salai is the model for Leonardo's painitng of St John the Baptist. Do a search for the image if you'd like to know what he looked like.