Since Romeo and Juliet was not an original story with Shakespeare, in fact it was widely known, he could safely assume playgoers would already be familiar with the story and characters. So there is very little explanatory material describing who the characters are and their reelationaships.
I want to enumerate the major characters and some helpful information regarding how they are interconnected.
Juliet: 13 years old, and an only child. Interestingly, the number 13 does not occur in the play. She is always said to be almost 14. The number 14 pops up in a number of places. Additionally, there are several prominent 14 line sonnets in the script.
Capulet: (her father) seems to be at least 50 years old (there is a line about it having been 30 years since he last attended a masque) Fifty seems old for having a 13 year old only child. Perhaps other children died? There are cryptic references in the text suggesting this may be the case.
Capulet’s Wife: (never called Lady Capulet) Could she be 28 ears old? At one point she claims to have been 14 when Juliet was born. If she was 14, her husband would have been 36. Or is the story about being 14 just a way to convince Juliet that she is old enough be married?
Tybalt: Juliet’s cousin (his father was Capulet’s brother). He is the character in the play most avidly pursuing the feud between the families.
[Rosaline] Romeo is said to be in love with her at the beginning of the play. She is mentioned and is said to be in attendance somewhere at Capulet’s masque, but she has no lines in the play. She is apparently Tybalt’s sister, which would make her Juliet’s cousin.
Prince Escalus: the hand of the law. The name Escalus suggests the scales of justice which he administers.
Romeo: the only son and apparently only child of the Montagues. In the Italian source he is 20 or 21, but Shakespeare gives no hint of his age. Many commentators believe he is quite a bit older than Juliet. Compared to the amount of information we are given about Juliet, Romeo is almost a cipher, at least as far as family and past life are concerned. We initially know almost nothing about him beyond his initial swooning infatuation with Rosaline.
Montague and Montague’s Wife: are small parts with few lines of dialog. Compared to Juliet’s parents, there is little interaction with their child.
Benvolio: Calls Montague uncle and Romeo cousin. These may be honorific, or this may be an actual relationship.
Mercutio: is apparently related to Prince Escalus. Perhaps his name is meant to suggest he was born under the influence of Mercury: lively, quick-witted, volatile. He is plainly a literary descendant of John Fallstaff of Shakespeare’s two previous plays Henry IV parts 1 and 2.
Nurse: has been with Juliet her entire life. She is nearly as verbally adept as Mercutio. She serves a Juliet’s confidant, at least early in the play.
Paris: Juliet’s suitor, apparently a member of the Escalus family. Perhaps this is why Juliet’s social-climbing mother is pushing for an early wedding.
Friar Laurence: by lines of dialog, is the 3rd longest part in the play. This is somewhat curious in that few people in Shakespeare’s Reformation England of 1585 would have had any contact with monks or priests.
Chorus: this is a role inherited from Greek drama. Chorus is a character who serves a role somewhat like a voice over would in a movie or TV show, providing commentary. Chorus appears at the beginning of acts 1 and 2. In each case chorus speaks a 14 line sonnet.