The thing is- all of the other blurbs use pronouns. Masaru and Jotarou are referred to with “he”, and Kotoko and Monica are referred to with “she”. Nagisa’s blurb, however, specifically avoids usage of any pronouns and rather repeats their name instead. I believe Hajime Isayama does the same thing with Hanji in the Attack on Titan manga, since in the mangaverse Hanji is intended to be of ambiguous gender. Since Nagisa’s blurb so specifically used no pronouns while the other blurbs did, it’s safe to assume that Nagisa is of ambiguous or non-binary gender.