The sketch seen here is by the Lombard artist Francesco Melzi, Leonardo's pupil, companion and primary heir. Melzi became an apprentice to Leonardo in 1508, during the latter's second stint in Milan, and remained at his side until Leonardo died in 1519.
The fact that Melzi, and the aptly named troublemaker "Salai" ("Offspring of Satan") were both protégés of Leonardo - regardless of their respective artistic talents or lack thereof - has caused speculation over the years. We all know how tongues love to wag. Were they apprentices or something more? Honestly, no one knows this except the above men, all long dead, never uttering a peep while they lived and leaving no tell-all diaries. I have gathered a few thoughts about Leonardo's potential homosexuality here, though, and offer further sources for the truly curious.