Many themes can be found in Romeo and Juliet, among those are love vs. infatuation; uncontrolled, violent, passionate emotions vs. rational thought; consequences of holding a grudge; consequences of violence and hatred; and even fate vs. free will. While we are limited to space and won't be able to discuss what characters represent all of those themes, below are a couple of ideas to help get you started.
Almost all the characters, to some extent, represent the theme of uncontrolled, violent, passionate emotions. In fact, it's one of the most dominant themes. However, Romeo certainly portrays the theme very well. We see him portray the theme in the very first scene in which we meet him. We learn that he is obsessing over Rosaline's rejection of his affections and allowing her rejection to absolutely crush him. We learn that for quite some time he has been staying out all night and is seen every morning at dawn standing under a grove of trees in one part of town, crying. We can presume that the trees are in the same part of town in which Rosaline lives and that the grove of trees offers a view of her house, possibly even of her bedroom. As Romeo's father phrases it, "Black and portentous must this humour prove / Unless good counsel may the cause remove" (I.i.137-38). In other words, Lord Montague is concerned that Romeo's sorrow may become harmful should he be allowed to continue feeling the way he does, and he needs someone's good advice to help him out of it. While Romeo does stop pining for Rosaline, he never stops feeling his intense, passionate emotions; he just directs his feelings at new targets, and it is his lack of control over his emotions that eventually leads to his young death. We see him act with uncontrolled, violent emotions when he so suddenly decides to marry Juliet, and we see him act with uncontrolled, violent emotions when he avenges himself on Tybalt for killing Mercutio; both decisions help lead to his death.
The best character that portrays the theme of rational thought is Benvolio, Romeo's friend, cousin, and dramatic foil. Benvolio is a very rational-minded peace keeper. We especially see Benvolio using his rational mind when he tries to persuade Romeo to listen to his advice and forget about Rosaline. Later, we see Benvolio acting very rationally when he begs Mercutio to get off the street, knowing that if they run into any Capulets, another brawl will be started, as we see in his lines:
I pray thee, good Mercutio, let's retire.
The day is hot, the Capulets abroad.
And if we meet, we shall not scape a brawl.
For now, these hot days, is the mad blood stirring. (III.i.1-4)
Had Mercutio listened to Benvolio's rational advice, Mercutio would have spared his own life, Tybalt's, Romeo's, and even Juliet's, showing us that the use of or lack of rational thought is an extremely central theme.