What characters are mostly involved with the theme of love versus infatuation in Romeo and Juliet?

The characters in Romeo and Juliet who are most involved with the theme of true love versus infatuation are Romeo, Rosaline, and Juliet. Although Romeo and Juliet have come to symbolize "true" love, the action of the play does not allow for that interpretation. It is apparent that both Romeo and Juliet are in the throes of strong infatuation. Romeo simply transfers his infatuation from Rosaline to Juliet.
As the play opens, Benvolio tries to get Romeo to attend the feast at the Capulets' home in order to get his mind off Rosaline, "whom thou so loves." He's convinced that once Romeo lays eyes on some of the other pretty women of Verona who will be at the party, he'll quickly forget about his "love" for Rosaline. Indeed, this is exactly what happens. Romeo sees Juliet and quickly makes his move. Before long, they've exchanged a little light banter and two kisses.
They stay up all night pining for each other and assuring each other of their love when they meet on Juliet's balcony. Juliet herself realizes that "it is too rash, too unadvised, too sudden" and calls their feelings "this bud of love." But as Romeo tries to leave, she more or less proposes to him, saying that if he wants to marry her, he should send word the next day, and she will meet him immediately for the wedding ceremony. After two meetings, they have committed to each other. Obviously, this is not enough time for true love to develop.
Paris's feelings for Juliet are even less developed. He seeks to marry her as part of an arranged marriage. Although Capulet first advises Paris to woo Juliet and win her heart, because of the way events transpire, this plan is dropped in favor of a wedding in a few days' time. Paris says nice things about and to Juliet, but he does not seem to be particularly infatuated with her. They have barely met each other. To him, the relationship is more like a business deal.
Therefore, Romeo and Juliet are the characters who display infatuation. True love is not really seen in the play, but the theme comes through as the shallow relationship of the two young people plunges ahead with disastrous consequences.