It's misapplied here, unless you buy into the "several girls with interest in one guy who is either only interested in one of them or isn't actively pursuing any of them at all" theory of "harem". It's not that he's "dense" or anything, either; he's been straightforwardly confessed to prior to the events of the manga and recognizes at least some of the girls' interest in him here. He's simply lived a life where he's been too overwhelmed by his circumstances to seriously consider romance and doesn't think he's at liberty to change that now.
While that might not be entirely inaccurate, her behavior and lack of understanding of the world outside her home and school can easily be laid at her father's feet. As middle volumes of the novel series make abundantly clear, she has absolutely no cultural knowledge whatsoever, nor has she been exposed to even classical literature of the sort that would be standard in a commoners' high school at her father's specific direction. She is not allowed even the simplest deviation from her father's demands. Her masking is part survival, part lack of any other option. At least up to now.
