。
She isn't the kind of person you think she is.
Literal
She [topic-は] you [subject-が] are-thinking [like-ような] person is-not.
思っている in resultative ~ている form expresses 'is currently thinking / has the impression that' — the opinion is held as an ongoing mental state rather than a passing thought. Compared to 想像する ('imagine, picture') and 考える ('think over, consider'): 思う sits between the two — more grounded than imagining, but less effortful than considering. It's the all-purpose verb for holding an opinion or impression.