That sentence is an example of parallel structure.
When there is repetition of a specific grammatical form in a sentence, parallelism or a parallel structure is produced. This occurs when you mantain the same pattern for every compared word or idea in your construction. This makes the idea easier to follow by the reader.
Assuming this is when they first meet, Boo is different from what they think because he is not the mean dangerous man he is rumored to be but rather kind and sweet.
