Monseigneur is a character. He’s also an allegory.
Wait, how can a character be both a character and an allegory? Well, Dickens describes Monseigneur as a member of the aristocracy. It becomes pretty clear, however, that "Monseigneur" also becomes a shorthand way for Dickens to refer to the aristocracy as a class.
For example, when the narrator spends a good portion of a chapter describing how Monseigneur takes his hot chocolate, we could be reading about one man. When we read, however, that "Monseigneur, as a class, had dissociated himself from the phenomenon of his not being appreciated," it seems fairly self-evident that we’re reading about more than one individual. In fact, we’re reading about an entire class of people (2.24.3).
Why the blurring between individual and class? Well, for one thing, it allows Dickens to describe an entire group of people rather quickly. Once we know how picky and self-satisfied Monseigneur is when he drinks his chocolate in the morning, we’re probably pretty ready to hate on him for the rest of the novel.
In some ways, that’s not a fair analysis. Charles Darnay is a monseigneur, if we get right down to it. But maybe the allegory becomes as important for the ways that it doesn’t fit as for the ways that it does. We’re ready to hate all aristocrats. They’re all bad. But that makes us…rather like Madame Defarge. Scary, huh?