Dark Ginny
As the series is named "Harry Potter and...," the center character would be Harry Potter and the world through his experiences. The lovely thing about fan fiction is that you may take a secondary or even lower level character and create a background that's been omitted. Well, really you could say even the main characters aren't really touched upon. We know very little about Hermione besides her obvious ability to know everything about everything. However, my personal favorite to write is the lovely but non-Trio based Ginny. This essay focuses on a Dark Ginny.
Writing Ginny as dark and slightly mad can be an adventure in itself. No one really knows what goes on in the girl's head after her possession by Voldemort. May be she wasn't affected but that's highly doubtful. The best example of the lasting effects is in OotP (US pg 499/500.)
"I didn't want anyone to talk to me," said Harry, who was feeling more and more nettled.
"Well, that was a bit stupid of you," Ginny said angrily, "seeing as you don't know anyone but me who's been possessed by You-Know-Who, and I can tell you how it feels."
Harry remained quite still as the impact of those words hit him. Then he wheeled around.
"I forgot," he said.
"Lucky you," said Ginny coolly.
"I'm sorry," he said, and he meant it. "So...so do you think I'm being possessed, then?"
"Well, can you remember everything you've been doing?" Ginny asked. "Are there big blank periods where you don't know what you've been up too?" Harry racked his brains.
"No," he said.
"Then You-Know-Who hasn't ever possessed you," said Ginny simply. "When he did it to me, I couldn't remember what I'd been doing for hours at a time. I'd find myself somewhere and not know how I got there."
As you can see, it's been almost three years since the end of her possession and she still feels the emotions at a pretty high level. At 11, how could that not affect her so radically? Then at 14 it would still be with her because she's around the Boy-Who-Lived all the time as a reminder. Over time, the feeling might be become more prominent since the imminent war is coming more quickly. Ginny has a bit of a temper - well, she is a Weasley - and that mixed in with her after effects could create a wildly entertaining character.
Who knows what she's thinking? Or feeling? We don't know a lot of the details so as writers we get the fun job of putting her character together, almost piece-by-piece. When I write the character, I tend to make her a little bit mad. A mixture of her mixed with Dru from Buffy the Vampire Slayer. I think if Ginny wasn't given the proper psychological treatment after the War - where the end of her torment will possibly come to an end since it's very likely the Dark Lord will fail - she has the potential to become Dru. Both were driven mad by means they didn't have control over.
Her mind will be a maze of confusion. That will become the best part of writing her. Her mind will be confused which allows for bizarre behavior. Say she kisses Harry in front of Luna - his girlfriend in the story - and doesn't realize the chaos she's caused. Or perhaps she kills someone because she thinks they're Voldemort come to get her again. A Dark Ginny is one that can be used for many plot devices. Use your imagination and twist your mind to fit a slightly mad girl.