Olivia is in Julie's class and is good at geometry, and when Julie hits problems, she will volunteer to help her, so they can become friends.
Introverted and shy, she doesn't have many friends.
How did they become a naturist or what is their relation to naturism?
Olivia isn't a naturist, and Julie doesn't open up about it, until much, much later in the story. As such, she remains oblivious to what is happening for most of the story.
She is just someone who helps Julie, and is friendly with her.