kristen stewart's character

Her full name on the basketball.. Samantha.

Then the scene when they go out to eat with the dad and she's wearing a dress.

She's supposed to be a girl. They make that clear in the movie. She's just a tomboy.

Tomorrow's just your future yesterday!

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Yes, she is a girl. This was mentioned quite late in the movie, around the "kidnapping", when I still thought Sam was a boy.

First of all Sam was short for Samantha. I recall at least one instance where an adult referred to Sam and her best friend as "girls". Although she had the dress and grooming of a tomboy and expressed interest in sports such as basketball, in one scene she wore a dress while dining out in a restaurant.

One point of interest was the scene where Randy tries to reconstruct his memory of his younger brother Johnny who died in an automobile crash. Randy accomplishes this by abducting Sam and later driving her to the location where his son died and uses her to superimpose his memory of his son by recreating in his mind the events that preceded the death of his son Johnny. Of course every time Randy called Sam Johnny, Sam would vehemently deny that she was his son. Yet she played along and understood the reason why she was temporarily kidnapped.