For much the same reason that there is but one planet to one orbit, so can there be but one such original… — Herman Melville

For much the same reason that there is but one planet to one orbit, so can there be but one such original character to one work of invention.

Two would conflict to chaos. In this view, to say that there are more than one to a book, is good presumption there is none at all. But for new, singular, striking, odd, eccentric, and all sorts of entertaining and instructive characters, a good fiction may be full of them. To produce such characters, an author, beside other things, must have seen much, and seen through much: to produce but one original character, he must have had much luck.

Source: Wikiquote · CC BY-SA 4.0