Did Shakespeare create the names of the characters found in Romeo and Juliet, or did they come from Arthur Brooke or someone else?

Well, it depends on what you mean when you say original
writer. He may not have come up with the story himself, or indeed the characters, but I
doubt anyone but him could have written it like that. He also probably modified the
story and characters to appeal to his audience as well. Or, on the other hand, he could
have heard of the plot of the story, ie, two people falling in love whose families' hate
each other, and made up all the characters and settings.