Tybalt is Juliet's cousin, and, from the first scene of the play, shown to be someone who intends on keeping the feud going, at all costs. When he enters Act I, scene i to find Benvolio attempting to part the Capulet and Montague servants, he decides that this is a perfect instigation to take this brawl to the next level. He says:
What, art thou drawn among these hearless hinds?
Turn thee, Benvolio, look upon thy death.
. . .What, drawn, and talk of peace? I hat the word,
As I hate hell, all Montagues, and thee.
And it is, in fact, Tybalt who is the only male character in the play without some sense of desire for at least periodically ignoring the feud. Tybalt is, no matter the occasion, always attempting to defeat the Montagues.
- He recognizes Romeo at his Uncle Capulet's feast and, even when Capulet demands that he let the boy alone, determines that he wil be avenged on Romeo for his mocking of his family's festivities.
- He challenges Romeo when he meets him in the streets, but is just as happy to duel the innocent Mercutio to the death.
So, Tybalt is a pretty two dimensional character. He exists in the play to drive the complications that are crucial to the plot of the tragedy. If he had not decided, in Act I, scene v, that he must be avenged on Romeo, then the crucial events of Act III -- the murders of Mercutio and Tybalt that lead to the banishment of Romeo -- would not take place.
Interestingly, Tybalt, as he speaks aside to the audience just before he exits Act I, scene v, shows all the dramatic markings of a villain:
I will withdraw; but this intrusion shall
Now seeming sweet, convert to bitt'rest gall.
And, just like an evil character who prophesies doom upon the events to come, Tybalt predicts and promises that there will be "bitter gall" to follow this superficially happy moment at his uncle's feast. And, like a perfect villain, he follows through to make sure that just this fate befalls the characters of the play.
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