What happens to this character at the end of Split (2016)?

The ending is left intentionally ambiguous and the viewer doesn't know for sure what happens next. She stays in the police car until the end of the scene.

She couldn't shoot her uncle when she was a kid, didn't tell her father about the abuse she suffered and allowed Dennis to kidnap her without resistance, but she finally shot the Beast, implying that she decided to stand up instead of being a silent victim. But that's about it, more than that would be speculation.

We get a final answer in Glass.

I don't have the exact script

She says to Kevin that she went to the police and spilled the beans.
Following this, she now live in a foster home, and looks happy.

At the end we see the main character, Casey, is hesitant to get out of the police car after the police officer tells her a couple times. This would presumably spark a red flag in the officer and make her wonder, despite the trauma the girl just went through, why she doesn't want to go home with her uncle. This is a sort of ambiguous ending in itself. But one could say that Casey would open up to the officer and she would be put under a new persons care. If she did tell the officer that she didn't want to go home with her uncle, then they most likely would ask questions as to why, and put her under someone else's care. I think that in the end she doesn't say anything because of the shock of what she went though, as well as the realization that she's escaped one "monster" just to be returned to another, and she has decide what to do next. Which is why she doesnt say anything to the policewoman. She's trusting that officer would understand that she doesn't want to go home with him.

Being a writer and screenwriter myself, I understand that M. Night accomplished exactly what he set out to do. Telling a story out to the very end every time isn’t necessary. If you watch closely, like one of the answers above pointed out Casey hesitated, however I’m going to take it a little further. Casey’s hesitation was accompanied by what is considered a “linger” look. In the look they focused on her eyes that gave the officer a clue into what was happening. If you look even closer, the officer’s facial and body expression changed in just the slightest way signaling some sort of recognition. Also, the wide shot at the end showed the statue of some sort of animal representing a beast and you’ll notice the male officer standing guard by the same police car Casey was in. So, M. Night tells the ending without verbally doing so, however he used non-verbal cues to get the message across.

We are left to wonder, and hope. Personally, I'd like to believe that she wasn't subjected to going back to her Uncle. Or, perhaps, that she gained the strength from surviving the kidnapping to put a stop to the abuse. Here's the problem though, I've learned (from personal experience) that your abuser never stops. It is part of who they are, fundamentally. As the saying goes, "A leopard cannot change their spots." So, I would like to think that this young lady was put in witness protection and never had to be around that "Uncle" again. I'm sure it's not be the story line, but it gives me comfort.