Man a Machine does not really have characters, given that it is a philosophical treatise rather than a novel or a short story. Nevertheless, we can point to several individuals and intellectual traditions which La Mettrie addresses and is in conversation with.

Theists

La Mettrie saves the greatest of his ire for theists. From the very beginning, La Mettrie voices a disdain for abstract speculation, but he finds the theistic tradition particularly noxious. Indeed, La Mettrie seems to voice a conviction that the world would be happier and better off if organized religion were done away with.

René Descartes

One of the most significant influences which La Mettrie interacts with is that of Descartes, and for all that he disagrees with Descartes on the subject of philosophy, he also voices tremendous esteem for Descartes's brilliance. To La Mettrie, Descartes understood the mechanical nature of man better than anyone else in his time. However, for all his accomplishments, Descartes also introduced the idea of mind-body dualism. Thus, even as La Mettrie esteems and admires Descartes (and even views him as an intellectual forerunner), at the same time he opposes the Cartesian tradition, arguing for a pure mechanical, materialist account of the mind, rather than the dualistic position that Descartes envisioned.

Gottfried Leibniz

To quote the International Encyclopedia of Philosophy, "Leibniz's universe contains only God and non-composite, immaterial, soul-like entities called 'monads.' Strictly speaking, space, time, causation, material objects, among other things, are all illusions." This advances a vision deeply opposed to the kind of materialism which La Mettrie advocates and represents a second philosophical influence which he is actively debating against.

John Locke

Leibniz, Descartes, and theism are the three most critical traditions which La Mettrie argues against, but he also mentions Locke from time to time. As far as Locke is concerned, it's worth noting that, as compared to the theists and Leibniz, Locke shares significant common ground with La Mettrie. Locke is particularly significant in the field of epistemology for being one of the key advocates of empiricism: the belief that knowledge is shaped by sensory experience (a viewpoint which favors scientific inquiry and is well in line with La Mettrie's own materialistic worldview). In addition, La Mettrie invokes several names and examples associated with the practical experimentation of the Scientific Revolution.