On the surface, Julia appears to be a vigilant and emphatic believer and follower of the Party and all of its rules, programs, mantras and ideals. She follows through with all activities and assignments she is given, she takes the time to join extra functions like the Anti-Sex Junior League. She plays the part so well, in fact, that Winston despises her for it, and even fears her a bit for reminding him of the exact type of person that would report him for ever having doubts.
However, under the polished exterior, it is quite a different story. She is rebel, and one who is entirely cynical and non-believing in the Party having any sort of sincerity, truth or goodness to it. She hates the Party, and goes out of her way to hold a fist of defiance against it. However, she is smart; she does so only in ways that are very careful, and allow her to not get caught. She is a thoughtful individual, one who has really thought about the meaning and purpose of the Party, and she is also incisive, seeing through its hypocrisy much more than Winston even does himself. She is passionate, opinionated, insightful, incisive, brave and aggressive. She seeks what she wants and goes after it with full force. She is confident and assured.
Julia's character is a necessary part of the book; without her, Winston's rebellion might not have happened. She is the catalyst that sets everything in motion. I hope that helps; good luck!