When the author uses "body like a harp," he is referring to how the protagonist has let his crush on Mangan's sister totally consume him and how she penetrates every fiber of his being. He has forgotten everything but her and neglects his schoolwork, etc. He can think of nothing else but her. It can also refer to how, if she cared to do so, she could completely manipulate the protagonist and do with him what she wishes, as a harpist does with his/her harp. The drawn shade might represent a sense of being "closed off" or not being open to something, for example. Mangan's sister hasn't shown the protagonist that she is interested in him as a "boyfriend"; however, he is enamored with her because she took the time to speak with him several times at leisure.