Have you read the Divergent series by Veronica Roth? If you haven't, you need to stop what you're doing and go read it - seriously. Better yet, listen to Emma Galvin read it to you on Audiobook.
Tris, the main character, has just turned 16. She lives in a dystopian world in Chicago's future and must choose which of the five factions she wants to belong to. The tagline for the book is "One choice can transform you" and so it does... Tris begins a journey of self-discovery that leads her to "ordinary acts of bravery, in the courage that drives one person to stand up for another".
Tris, Four, Christina, Will and the others are the types of characters that stay with you.
Excellent. I wish the series had never ended. However, if you haven't read them, you're in luck - you won't have to wait forever for the sequels to come out. :)