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: Evie was based off several people in Nikki Reed's life.