Romeo's main characteristic is that he is in love with being in love. His other main trait is his way with words. At the masquerade ball and during the balcony scene, he knows how to woo with language. In fact, these characteristics are so powerful that today a "Romeo" means a smooth-talking seducer.
Of course, Romeo is nothing but utterly sincere in his love for Juliet, and he shows he means what he says when he marries her the day after they meet.
Another characteristic of Romeo is his impulsivity: he falls in love quickly. Further, and no doubt an aspect of being an adolescent, he very much is in a hurry. Everything has to be now for him, which explains the very hasty marriage—and his rush to obtain poison when he hears (wrongly) that Juliet has died.
If we were to further develop his character, we would note he has the security of loving parents, especially a loving father who is concerned as the play opens with Romeo's mooning about. He also has a gift for friendship as well as love. We can therefore imagine, that having been raised with a sense of inner security, he would have been a good father to any children he and Juliet bore. With his personality and gift for wooing people, he could have, as he matured and tamed his impulsivity, become an important force bringing together the two feuding families—without having to die.