I don’t believe you’ll ever know everything about your characters. Writing is a process of discovery and, just like with real people, characters are always growing and changing.
Still, you should aim to get a decent understanding of your main characters before you get too far into the story. One of the things that annoys readers is when a character doesn’t act “true” to his or her self, and that sometimes happens because the writer changes something along the way and doesn’t go back to make sure the character is consistent throughout the story.
Chose 3 life experiences that have helped shape your character: Her parent’s divorce when she was 10, her older brother’s car accident that left him in a wheelchair, and her unresolved relationship with her former best friend/boyfriend stealer. Consider how each of these experiences has made her the rebel/phobic/green-eyed monster she is today. Then go through your story and make sure that this is the same girl throughout.
Your rebel isn’t going to just do the dishes when her mother tells her to, she’s going to bitch and maybe stomp out of the room; although she may gladly wash dishes for her favorite grandmother, who has always treated her like a princess. Your phobic isn’t going to just get into a car with any boy, she’s going to think about the lack of airbag and grab onto the seat cushion every time he takes a corner too sharply. Your green-eyed monster (who is hopefully not cliched enough to actually have green eyes) isn’t just going to take her little sister to the recital that mom and dad are too busy to go to, she’s going to sit there the entire time thinking about how she could have actually made it to competition level in chorus if someone had bothered to get her to rehearsals.
At each point of your story, your character is who she is. She is that little girl whose dad left her, even when she has a perfect boyfriend. She is that worrier, even when she decides to walk on the train tracks with the mysterious could-be boyfriend. She is that jealous girl, even when she befriends the school slut who once had a thing with her boyfriend.
Keep in mind that Carlie is always Carlie, and you’ll do your writing (and your readers) a big favor.