Who is the speaker of The Prologue in Act I of Romeo and Juliet?
The speaker in this Prologue is not a character in the play. Instead, the speaker is "the chorus" -- probably a group of the actors who are about to perform the play.
The reason I say that the chorus is not a character in the play is because, in the Prologue, the chorus is explicitly speaking to an audience about a play. The chorus talks about where it is that they "lay their scene." And the chorus asks the audience to pay attention with "patient ears."
This makes it clear that they are acting not as characters in the play but as something outside the play. They are not pretending to be other people. Instead, they are a group of people saying "here's what the play is going to be about, please listen and enjoy."
The chorus acts as a narrator. You will find the chorus only at work before the first Act and in the beginning of the second.
During Shakespeare's time, when plays were performed on the new stage of the Globe Theater, props and special effects were very minimal. The Prologue served that purpose of filling in the blanks ahead of time so that when the actual play began and got underway, audience members could refer back in their memory to the introductory material they heard.
The chorus gave a sort of road map to the audience for what was about to happen.
You could also argue that the chorus' speaking parts are Shakespeare's opportunity to talk directly to his audience about what was going on.