The Spirit of the Laws by Montesquieu isn't a work of fiction, so characters don't appear in it the same way they appear in a novel. There are actors, however, in the political science sense of the word, and if that's what you mean by characters, then there's a lot to discuss. We won't recapitulate Montesquieu's arguments here, we'll preview them. If you want to know about them in detail, you should read the excellent study guide available on this website.
Actors are people who have what we call agency. They do stuff. They're active, as their name implies, not passive. We don't have to know who they are individually. We just have to recognize what they do and what the causes and effects of those actions are. Characters in a novel just show up. They may or may not act, but they're part of the story. Here, actors create political systems.
If you read Montesquieu critically, you realize that The Spirit of the Laws is at least as much a social commentary as a work of political philosophy. Read in between the lines of his theories of republics, monarchies, and despotisms, and you'll find that the actions of citizens, and the context in which those actions happen, matter. Another way of saying this is, different people, speaking of whole societies, will create for themselves different governments. The actors matter. They're critical.
People who love virtue, says Montesquieu, tend to give themselves republics. Those who love honor plumb for monarchies. Fearful people suffer despots. Which are you? Which are we? If you're reading critically, you'll understand that societies come to fit one of these categories because of the material, natural and historical conditions in which they exist. So, actors are critical, but they're not everything.
Those material and other conditions can have a large, sometimes determining influence on a society's political culture. These aren't actors, because they don't have agency, but they do exert power. Really, if you want to understand the "characters" in Montesquieu's work, you have to understand people and their environment.