Lili, from Legend.

Running through the meadows to go see Jack, she stops by Nell's house. She sneaks up on Nell doing laundry and breaks the clothes line, distracting Nell so Lili can do a quick B and E and steal Nell's cookies. Later she meets up with Jack and is like "I brought you a present. I made them myself!" and gives Jack the stolen goods. Now, you have to understand that Jack is in love and when you're in love you make some pretty bad decisions. Jack's like "Lili, I have gift for you now. I want to show you the unicorns. The most precious, pure, and important thing in the world, but under one condition: we hide and observe from a distance so that we don't disturb them." So Lili runs up and the touches one of them. Later Lili and Jack are at the top of a waterfall and she takes off her ring and says "I'll marry the man who finds this ring" and throws it over the waterfall into the lake below. Jack with his great decisions, jumps off the waterfall without hesitation, and the whole time Lili is screaming and crying "OMG why would you do that?!". Jack, gets the ring but almost drowns. There are other instances of Lili being an ass, but I'm starting to get mad typing.