Tybolt is labelled a 'saucy boy' by Lord Capulet, revealing the irony that although Tybolt taunts Romeo about being a boy, Tybolt himself is chastised as one
Mercutio: 'Thou consortest with Romeo, what, dost thou make us minstrels? And thou make minstrels of us, look to hear nothing but discords: here's my fiddlestick; he shall make you dance. Zounds, consort!'
Signify different levels of formality - 'thou' was used between friends in lower classes or to show social superiority/scorn, 'you' was used between friends in the upper classes or to show respect
Tybolt only appears three times in the entire play, but his impact is significant - Romeo's murder of Tybolt is the climax of the play, leading to the downward spiral that ends with Romeo and Juliet's deaths