In Romeo and Juliet, they are many diverse and different characters. Some are kind, and others are mean. The character that I find the most interesting by far is Marcutio. He is in the Montague house, and very good friends with Romeo. A way to describe him could simply be attention-seeking and reckless. But I think there is something that rings very true about his character. He's not like Benvolio, who is simply peaceful. Nor is he like Tybalt, who is angry. He is his very own person.
With Marcutio, there is always that undercurrent of recklessness and restlessness. He is happy to pick a fight with the Capulets, or crash their forbidden party. He is a funny character, always mocking Romeo for his poetic sadness, and overall lightening the mood of the play. Perhaps that is why when the play takes a darker turn, Shakespeare felt the need to kill his character.
But underneath it all, there is something deeper with him. This is evident in the way in which he cares about Romeo. Thought he teases him for his brooding, it almost seems like he is taking care of him at the start of the play. He takes him to a party so that he can move on from Rosaline, and when he is not there talks to Benvolio about the pain that the "wench" Rosaline has put Romeo through. Marcutio is not just a comic relief. He is the friend of Romeo, and the piece that signals a change in the mood of the play.
Marcutio is not just 2-d, and maybe we would have seen more of that had he been around for the end of the play. Maybe he is not the most common type of person, but I think there is definitely something real about him.