While we are limited in space to discuss adjectives for each character, here are a few ideas to get you started.
Two adjectives to describe Romeo are irrational and emotional. We especially see his irrational emotionalism in the very first scene with respect to his response to being rejected by Rosaline. Romeo's father describes Romeo as being seen each morning at dawn in the same part of town, crying under a grove of trees, as we see in Lord Montague's lines:
Many a morning hath he there been seen,
ith tears augmenting the fresh morning's dew,
Adding to clouds more clouds with his deep sighs. (I.i.127-29)
We can assume that Romeo has been staying out all night to be in the same part of town where Rosaline lives and that the trees offer a view of her house, possibly even her bedroom. As we can see, crying all night long over Rosaline is certainly a very emotional reaction; we can even call it somewhat irrational. Even Benvolio shows his belief in the irrationality of Romeo's actions when he begs Romeo to listen to him and to forget about Rosaline.
An adjective describing Benvolio is sensible. In fact, Benvolio is one of the most sensible, rational, practical characters in the play. Not only do we see his good sense when he begs Romeo, "Be rul'd by me: forget to think of her," we even see it earlier in this very first scene when Benvolio tries to break up the fight between the servants (I.i.227). As one of the only sensible characters, eager to create peace, Benvolio tries to separate the battling servants, saying, "Part, fools! / Put up your swords. You know not what you do" (59-60). However, Tybalt misjudges Benvolio's actions, and believing Benvolio has started a fight with a bunch of useless servants, challenges Benvolio, making the fight escalate into an all out street brawl.