Idealistic. She blindly accepts what Romeo says, because she wants it to work out. She trusts that the Father's plan to fake her death will work, and that Romeo will be there waiting for her when she wakes up. She tends to only see the good side of the situation, and then is devastated when it doesn't work out perfectly as she believed it should.
Naive would be an apt description. She has no experience whatsoever with love or men in general, yet she immediately believes Romeo's declarations of love and honor. Yeah, he does intend to marry her, but how exactly can she trust that? She rushes into this relationship (and marriage), never questioning that maybe this might be a bad idea. If I could get another word, I would definitely say idiotic.
Determined. She is determined to marry Romeo in spite of her families' issues with his. This is even more true as other events occur...Mercutio and Tybalt die, Romeo is banished, Paris is chosen as Juliet's husband. Then she, the nurse, and the Friar hatch a plan to get the two of them together.
The one word that would best describe Juliet would be impetuous. Impetuous, if you don’t know, means that she does things without thinking and rushes too quickly into situations and acts. This can be used to describe her from the very beginning of Act 2 when she tells Romeo that if he truly loves her than he must send word to her Nurse the next day of their marriage arrangements through the very end of the play when she rushes in to taking the potion made by Friar Lawrence. If Juliet had not been impetuous, the entire play would have taken a different turn. This character, just as Romeo, did not think about any of the actions that she took throughout the play, and therefore, this is not only the best word to describe her, but it is also her character flaw or tragic flaw.