Julia reverses our expectations of what a woman would be like in that society. She is a contradiction: on the one hand, she is a walking advertisment for the anti-sex league; on the other, a promiscuous woman whose goal is to spread corruption through the party by having sex with as many party members as possible. Unlike Winston, who appears to be fumbling his way into revolutionary acts, she has a system and a plan. She's instigates the affair with Winston and she has actually worked out how to avoid detection (or so we think). Winston tries to be invisible, but Julia makes herself as visible as possible in her community so that on the outside she is everything the party could want. But on the inside, at least for much of the story, she is her own person. I think she is memorable because of this.
Orwell does a great job of writing 1984 in such a way that we connect very closely with Winston. Julia is significant and memorable because she is those things to Winston and we are so closely connected to Winston's thoughts and actions that we become equally close and connected to Julia because she is such a prominent figure for him. Orwell provides a great description with the use of powerful imagery for us. We see her dark hair, red lips, and matching sash and at first we hate Julia along with Winston and we feel his fear when he sees her outside his shop and contemplates murdering her. Then we are intrigued with Winston when she gives him the note that reads "I love you." We grow to like Julia with Winston because she is against the party and attempts to sabotage the Party's ideals every chance she gets. Even though she betrays Winston in the end, we feel nothing about her along with Winston- we understand that sometimes they threaten you with something so severe that you have to betray the people you love.