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Class F student who speaks like an old samurai and happens to be a great drama club member, but has mediocre grades. On top of that, everyone in school mistakes him for a girl.
Hideyoshi happens to be a very good actor and drama student, but his skills in academics are very low, and therefore he ends up placed in Class F with Yuuji, Akihisa, and the others. Compare this to his twin sister, who has the opposite attitude: a no-nonsense girl who studies very hard and doesn't have much time for drama or extracurriculars, and as a result tested into Class A.
A running gag throughout the series is that everyone seems to think that Hideyoshi is a girl, from his fellow classmates who buy and sell lewd photos of him in girly costumes to the unnamed shopkeepers who keep selling him feminine outfits. During Ep. #6 of the anime, Hideyoshi even had his own separate bathroom and onsen outside of the girls- and boys-only rooms. The only person in the story who ever thought Hideyoshi was a boy was Akihisa's sister Akira, but that was only because she refused to believe that Akihisa would befriend any females except for her.
In a Japanese poll of favorite light novel characters, Hideyoshi was somehow able to win both first place among all male characters and tenth place among all female characters.