Cooperative- Juliet agrees to consider Parris as a potential suitor at her father's party, even though she says she's not particularly ready to consider marriage.
Affectionate - The person Juliet is most affectionate with is the Nurse. They apparently tease one another, and Juliet literally hugs her when she comes back from talking to Romeo.
Sincere - When she tells her father she has not even thought of marriage, Juliet exhibits sincerity.
Mature - She is immature and impulsive most of the time after she meets Romeo, so any maturity is displayed before that. (See Sincere and Cooperative.) A case may be made that after she gets the potion from the Friar and becomes an obedient daughter (knowing she's not going to have to actually marry Parris) she is showing some maturity. When Juliet does not, for once, share her plan with the Nurse, she exhibits mature behavior.
Courageous - When Juliet takes the Friar's potion, she show courage--though she's young and may not even recognize the dangers inherent in such an implausible plan. Certainly when she buries Romeo's dagger in her breast she shows extraordinary courage and conviction.
Heroic - See Courageous.
Decisive - Choosing to marry Romeo after just a few hours' acquaintance shows decisiveness, as does her determination to carry out the Friar's plan.
Devoted - Juliet is clearly devoted to the Nurse, her father, and Romeo--though she breaks faith with all but Romeo throughout the course of the play.
Loyal - She is loyal to both her beloved cousin Tybalt and her husband Romeo, though her loyalties are tested when one is killed by the other.
Faithful - See Loyal.
Passive - Agreeing to consider a man she hasn't even met as a husband is the epitome of passive.