Hologram of the Renaissance genius in Janeway's Florence holodeck program, who became her confidant and was once stolen along with Voyager's technology.
Janeway's holodeck program recreated the workshop of Leonardo da Vinci in Renaissance Florence, where she spent time as his apprentice. Played by John Rhys-Davies, the hologram first appeared in Scorpion, when a conversation with him helped Janeway find a way to deal with the Borg.
In Concerning Flight, alien thieves stole Voyager's computer core and the Doctor's mobile emitter, which was running the Leonardo program at the time. On the alien world, Leonardo believed he had reached the New World. Janeway found him, and the two escaped their pursuers by flying his glider to safety.