The legend of Romeo and Juliet, including all the characters we see named in the play, actually has a very complicated history. The first person to record the story of two star-crossed lovers who die young in 14th century Verona was actually Luigi da Porta, who published his version in 1531. Da Porta's version of the story is entitled what can be translated from the Italian as Newly Found Story of Two Noble Lovers and tells the story of Romeus and Giulietta. His is thought to have been derived from a short story written by Masuccio Salernitano; however, da Porto added the characters that were later used by Shakespeare, including the questionable friar who creates a sleeping potion. It is also known that Shakespeare also used several novellas written by Matteo Bandello as a reference for creating his play. Finally, Arthur Brooke wrote the first English version of the legend with his poem The Tragic History of Romeus and Juliet, which was published in 1562. Shakespeare used all of these versions as well as others to create his play and the characters in Romeo and Juliet.
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.