。
She isn't the kind of girl you imagine.
Literal
She [topic-は] you [subject-が] imagine [like/the-kind-of-ような] girl is-not.
~ような modifies a following noun with a clause: 君が想像する ('you imagine') becomes 君が想像するような女の子 ('a girl such as you imagine'). The literal force is similar to English 'the kind of N that...' but with the relative-clause-like phrase placed before the noun. From the cluster of 'thinking' verbs (思う, 考える, 想像する), the choice of 想像する ('to imagine') here implies a fanciful or projected mental image, more vivid than the deliberative 考える or the simple 思う.