There are several characters that make up the supporting cast that help move the plot forward in Romeo and Juliet. I'll discuss a few here:
Benvolio is a sincere, good natured young man, who being Romeo's cousin and close friend serves as Romeo's confidant who hears out his problems, which often have to do with Romeo's love interests. Benvolio's advice is always upbeat and encourages Romeo to move foreward in his life and not get overly depressed over a girl. In his most significant piece of advice, he tells Romeo to attend the Capulet masquerade party in order to meet other women. Romeo is hesitant and pesismistic about the venture, but he agrees. This becomes essential to the plot because this is where Romeo meets Juliet and their whirlwind romance begins.
Lord Capulet appears to be a sober and understanding father, who cares deeply about his daughter and is interested in her feedback regarding her future desires, unlike many other fathers during that era. However, Tybalt's death greatly affects his rational and progressive thinking, and as the head of the household, he feels it is his duty to take immediate action in order to stop the grieving throughout the Capulet family. His rush to judgement forces Juliet into marrying Paris, putting Juliet in a predicament that leads her to fake her death, pushing the plot on toward the bloody final action and resoultion.
Tybal plays the role of Romeo and the rest of the Montagues' main antagonist. Tybalt sees himself as an honorable young man with the greatest loyalty to his family and, therefore, maintains the greatest hate toward all Montagues. His constant acts to defend his family's honor and to commit violence against the Montagues continue to fuel the feud despite the efforts of the Prince, Lord Capulet, and others' attempts to quell it. His insistence to fight Romeo in Act III is at least partially responsible for his fight with Mercutio that proves to be fatal for the latter. This prompts Romeo to give in and fight Tybalt, which ends in Tybalt's death and Romeo's banishment from Verona. The killings and banishment put an end to the Friar's hopes that Romeo and Juliet's union can bring peace to the Montague and Capulet families with no further bloodshed. It also puts Romeo and Juliet in a situation where they are facing life apart from one another, which allow them to consider the most desperate means to be together.