Who is Leonardo de Quirm from Men at Arms: A Novel of Discworld and what is their importance?

Leonardo de Quirm, loosely based on Leonardo de Vinci, was a member of the guild of Alchemists. Responsible for creating wild, outlandish, modern inventions, Leonardo is taken in by the Patrician, where he is kept in a locked room. Leonardo is free to work away at his heart's content on his inventions. Leonardo is a highly dangerous man, as his inventions have the power to shape history. Although, his inventions rarely work well, they display the seeds of the modern era that fascinates the Patrician. Leonardo is responsible for creating the 'Gonne' (or 'gun'), as an exercise in the study of rocketry and ballistics, and it is his invention that, when stolen, forms the basis of the entire plot of the story of Men At Arms.