When Keira was a freshman in college back in England (when she met Morgan), Charlotte was her flatmate.
Later in the book though she talked about living in the dorm so this appears to mean that Charlotte was Keira’s dormmate.
She isn’t really mentioned much in the book, just in an off-handed reference when Keira is talking about what happened to her in regards to Morgan.
You later find out that Morgan killed Charlotte, making it look like a suicide.