Armin is indeed a boy, but he is a feminine boy, hence the fact he has a female voice actor because his voice is quite high, and is too high as such for a male Japanese voice actor to voice him because there vocals are low.
there proofing that Armin is a feminine boy. basically a secret shota. XDD
I thought Armin was a girl from the start! This is an old 質問 and maybe a non-issue at this point but I just started the series on Netflix. Only a casual アニメ watcher(I'm an old lady) Gender-neutral's ok too. I just thought I might be foreseeing an upcoming plot twist, guess not.