Monadology is a philosophical treatise and therefore doesn't have characters in a conventional sense, except that despite the relative brevity of the work, Leibniz does refer to people who were his contemporaries or predecessors in philosophic thought.

Before getting to those, however, it might be better to convey what he means by a "monad" and state whether or not what he specifies by that term might be considered a "character." A monad is an indivisible entity, a kind of "atom," except that Leibniz is not referring to a scientifically verifiable thing but instead is trying to describe the makeup of the universe as it relates to his own philosophy and, ultimately, his theological views. A monad is not a "character," but given its centrality to Leibniz's thinking, it is more important than any actual characters he can or does describe. His explanation of the existence of the monad seems to me a way of both proving God's existence and also proving God's perfection. Though Leibniz was, at least in some sense, a predecessor of the Enlightenment, his philosophy was also derided by no less an Enlightenment figure than Voltaire, famously in the character Pangloss in Candide. Leibniz himself refers to Rene Descartes (1596–1660) in Monadology, asserting that Descartes and his followers (the Cartesians) misunderstood the nature of "perception" and disbelieved that a "perception" was something a person could not be aware of as it was happening. In Leibniz's view, Cartesians believed that the only indivisible entity was "the mind," and Leibniz considers this to have been an error. He relates this apparently to the "error" he alleges was committed by Aristotle and his followers, in their belief that 1) the soul is separate from the body and 2) the soul is mortal.

A lesser figure in the history of philosophy whom Leibniz mentions is Pierre Bayle (1647–1706). Leibniz refers to Bayle's Historical and Critical Dictionary, specifically to the article "Rorarius" in it, and to Bayle's having taken issue with Leibniz's own view of the nature of God. Bayle was a philosopher of French Huguenot background who was critical of rationalists—not only Leibniz but others such as Descartes and Spinoza—who tried to use reason to prove the existence of God and to describe God's essential qualities. The fact that Leibniz mentions him more than once in Monadology would indicate that much of his, Leibniz's, concern is to establish a non-faith-based view of God, which a Calvinist such as Bayle would have been opposed to. Though Leibniz's ideas expressed in Monadology might seem an analogue in rationalist thought to the standard beliefs of Christianity, we can see that at least in his time, Leibniz's philosophy was highly controversial and was opposed by both religious people and by freethinkers.

In summary, we might conclude that Leibniz himself is the principal "character" of this treatise. Like most philosophers, he puts his own rational brilliance on display and very self-confidently propounds his views that he genuinely believed did form a strikingly valid explanation of God and the cosmos, using the concept of the monad as his basis for demonstrating this positive and optimistic philosophy.