。
She isn't the kind of person you take her for.
Literal
She [topic-は] you [subject-が] are-considering [like-ような] human-being is-not.
考える ('to think, consider, contemplate') implies a more deliberate weighing than 思う or 想像する — closer to 'consider' or 'reckon' in English. And 人間 ('human being, person') has a heavier philosophical weight than the everyday 人 ('person'); it tends to come up when someone's character or moral substance is in question — 立派な人間 ('a fine human being'), あの人は人間じゃない ('he's no human being / he's a monster'). Together, 考えている人間 frames the (mis)assessment as a serious judgment of character.