The protagonist of a story is the main character, the character in which all of the events of the story surround. The protagonist can also be called the hero of the story. As Romeo and Juliet is a tragedy, the protagonist, or hero, will also be called the tragic hero. According to the standard, accepted definition, a tragic hero is a person of high social position, such as a prince or a lord's son, who is generally noble and virtuous, but also has some fatal character flaw that leads to his demise. Romeo certainly fits the definition of a tragic hero as well as a protagonist. Any quotes that will help portray Romeo as the protagonist as well as the tragic hero will show how he is central to the story line as well as what his tragic character flaws are.
We get the sense that Romeo is central to the story line when we first hear about him and meet him in the very first scene. Immediately after the brawl, Lord and Lady Montague start worrying about their son, asking Benvolio if had seen him, and telling about how Romeo had lately been seen staying up all night long and crying. Since this is the opening scene, his family fretting over him shows us that he is a central figure. In addition, when Romeo finally does appear onstage, he makes a very astute observation about love and hatred that actually captures a central theme in the play, showing us just how important Romeo is to the story. Romeo makes the observation in the lines:
O me! What fray was here?
Yet tell me not, for I have heard it all.
Here's much to do with hate, but more with love.
Why then, O brawling love! O loving hate! (I.i.171-74)
The most important line in this speech is, "Here's much to do with hate, but more with love." What Romeo is observing is that the feud between the Montagues and Capulets is actually being caused by each family's own love of their own ideals and perspectives. Therefore, it is their own ideals and perspectives that are causing the hatred they feel towards each other. The families' love is creating hatred, leading to the feud. Hence, these lines depict the central theme of love vs. hatred, and since Romeo is speaking the lines, they show us that he is the central character.
Another good passage portraying Romeo as a tragic hero, having character flaws that will soon lead to his demise, can be found in the scene in which Romeo allows his friends to persuade him to crash the ball. Romeo has already expressed his hesitation in going. In fact, he states that he feels it is an unwise idea. He had a dream he considers prophetical that showed him something terrible would happen as a result of his crashing the ball, such as his early death, as we see in his lines:
... for my mind misgives
Some consequence, yet hanging in the stars,
Shall bitterly begin his fearful date
With this night's revels and expire the term
Of a despised life, clos'd in my breast,
By some vile forfeit of untimely death. (I.iv.113-18)
Since Romeo feels so passionate about not going, seeing it as dangerous, the fact that he allows his friends to persuade him otherwise shows us that he is very young and rash, which happen to be two character traits that lead to his downfall, showing us that he is a tragic hero. Not only do these lines portray him as a tragic hero through revealing his character flaws, they also foreshadow his upcoming demise, again, showing us that he is the tragic hero, or protagonist.