Julia is Winston Smith's love-interest and his ally in the struggle against Big Brother. She represents the elements of humanity that Winston does not: pure sexuality, cunning, and survival. While Winston simply manages to survive, Julia is a true survivalist, using any means necessary to conduct her self-centered rebellion. Her demeanor is that of a zealous Party follower, but just under that thin surface is an individual with unchecked human desires and a willful spirit, which ultimately results in her capture.
While Winston enjoys sex and intimacy, Julia is an outwardly sexual being and sleeps with Party members regularly — at least before she meets Winston. She does not do this to destroy the Party but to quench her own desires, and that is the fundamental difference between Winston and Julia. His rebellion is as much for future generations as it is for himself; her rebellion is purely incidental to her own desires.
Julia is far more intuitive and realistic than Winston. She understands the Party better than he does and is more cunning in the ways that she defies Party doctrine. While Winston is emotional about the Party and its potential downfall, Julia feels his wishes are merely fantasy and is apathetic to the Party's dogma. She busies herself with getting around the Party, unlike Winston, who wishes to attack the Party at its center.
Julia uses sex to attack the Party, but it is far less effective a weapon than love. When Julia and Winston fall in love, they commit the ultimate offense against the Party. Note that the couple was caught at their happiest moment, the moment where they let down their guard and felt like an ordinary couple. Both had been watched for years and could have been captured at any time. But not until their love was strong did the Party intervene. Separating the couple diminishes their effectiveness: As individuals they do not understand the party wholly, nor are they capable of resistance.
Superficially, Julia seems like an uncomplicated character. She functions as a sounding board for Winston, but she is far more complicated than that. Winston has real antipathy toward women resulting from the Party's indoctrination and from its stringent sexual codes. Winston can remember a time when affection was shown for affection's sake and is angry at women for what the Party has done to them. Julia does not follow these strict sexual codes and, in fact, breaks them at every opportunity. She shows Winston, who once imagined raping and killing her, that the Party cannot get to the most intimate places in a human being's mind; she is his proof that the feelings that he has been having are valid. Julia gives Winston hope, and it is the continuation of this hope that gets them both destroyed.