At the beginning, she was assigned to kill The Wolf. What was her obsession with having Ethan do it. Even at the end when she had the chance to kill The Wolf, she insisted Ethan do it, though she eventually pulled the trigger.
It was HER assignment, but she "recruited" Ethan to do it (in exchange for that drug...BTW where the heck did she get it from???). Then, she disappears for half the movie, only to show up again in a pointless scene...finally, she actually kills the Wolf herself...???
Honestly, the movie could've done without Amber Heard's character. She was so over-the-top, unnecessary, and really bad.
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"Could you please get your head out of your ass? It's not a hat!" - Pitch Perfect
Did you pay attention? Vivi told Ethan she had access to MANY things which means she had access to experimental drugs (the huge horse needle). Amber Heard was excellent in this. She was there as a sidekick! Come on, people, don't be so hard nosed about this. I've seen it seven times now. I enjoy it more each time. It's great and Kevin Costner did a terrific job in this as well.
I agree with most posters here, she was pretty annoying and kinda unnecessary. Not as annoying as the Albino though, that guy should be casted for every alien movie!!!