Ron's kid sister, Ginny, doesn't get a whole lot to say or do in this movie…up until the end when we find out that she's the one who opened the Chamber of Secrets.
From the first moments we see Ginny, she's an adorably shy eleven-year-old girl who's totally tongue-tied around Harry Potter. When she sees him sitting in her kitchen eating breakfast she turns and runs back upstairs. Ron's kind of over it because Ginny's "been talking about [Harry] all summer." Ron doesn't seem to get it but we do: little sister has a crush.
We just catch glimpses of Ginny sort of hanging around and—cough—writing in a book for the rest of the movie. It isn't until she's taken inside the chamber that we find out her whole back-story during the year. Tom Riddle explains inside the chamber:
TOM RIDDLE: Yes, Harry, it was Ginny Weasley who opened the Chamber of Secrets.
HARRY POTTER: No. She couldn't. She wouldn't.
TOM RIDDLE: It was Ginny who set the basilisk on the Mudbloods and Filch's cat. Ginny who wrote the threatening messages on the walls.
HARRY POTTER: But why?
TOM RIDDLE: Because I told her to. You'll find I can be very persuasive. Not that she knew what she was doing. She was, shall we say, in a kind of trance. Still, the power of the diary began to scare her. She tried to dispose of it in the girls' bathroom.
Poor Ginny. Talk about a rough first year at Hogwarts.
Okay, so Ginny's a total damsel in distress for Harry to save, right? Not quite. Even though she's being possessed by one of the most powerful dark wizards of all time, Ginny does try to fight back by tossing the diary away. In the end, Voldemort is stronger than her and Ginny can't get rid of him on her own, but she's doesn't just let go without a struggle.
Ginny will come into her own in the rest of the film series (and maybe that crush will start to become a little more mutual) so this story is just the beginning of Ginny's adventures.