Trivia: When Tracy is spinning on the playground equipment in the final scene, she is wearing the clothes that Evie wore when they went to the neighbor lifeguard Luke's house. This may be a call back to her missing Evie after she left or a representation of how Tracy's style changed over time. Also, when Tracy wakes up next to Mel in the end she looks disappointed. And I have come to think that she is upset because she wanted to wake up next to Evie again. (00:49:51 - 01:34:51)
Trivia: Mentioned on Commentary - the poster at the bus stop "Beauty is Truth" is a picture of Nikki Reed's (Evie) eyes on another woman's face. The picture is supposedly symbolic because each time we see the poster it is more graffitied/ruined, symbolising how Evie is making Tracy more out-of-control and dirty (e.g. introducing her to drugs and sex.).
Answer: Tracy acted like she had done everything Evie did before. She pretended she was already bad to become friends with them, so Evie thought she had found someone who was like her. Evie was definitely also in love with Tracy, but was also jealous since Tracy had a family who loved her. She wanted that. Yes she used Tracy, but she also cared about her.
I don't think Evie was romantically in love with Tracy. It was more about her wanting to *be* Tracy, with a mother like Tracy's. Also, she seemed to know that Tracy was only pretending to be "bad." I think Tracy's innocence made Evie sad, because there was a time when Evie was more like her.