Is Juliet a selfish character in the play?
I wouldn't call Juliet a selfish character because I don't think it is selfish to be in love and to put that love and the person one loves first. What should she have done differently: marry Paris? Such an action would have gone against her conscience; she knew that she was already married in the eyes of (her) God, and she believed it to be wrong to marry another (or to marry someone she didn't love).
One might argue that she was selfish not to be more loyal to her father and mother, but how loyal can we really expect her to be when her father threatened to throw her out and allow her to beg and starve in the street if she disobeys him? This is hardly the behavior of a loving parent that deserves his child's obedience.
One might also argue that taking her own life was selfish, but I don't think this is ever a compelling or appropriate argument. A person doesn't end her life because she is selfish, she does it because she is in terrible pain and sees no other way to end that pain. Further, Juliet wouldn't be in this situation at all if her family had not engaged in a ridiculous and violent feud with the Montagues over little else besides "family honor." If anything, it is their selfishness that causes problems, and not hers.