Hideri is a Crossdresser. Or to use Japanese terms (since this is a Japanese work..), a otokonoko.

1. ⁠Anyone who takes the "But he wrote girl in the resume" seriously doesn't realize he either was 1. purposefully trying to troll Dino or 2. thought Dino wouldn't hire him if he outright admitted he's male, or 3. both. As soon as Dino outs him as male, though, his reaction in the manga is "Yep! I'm a boy! ♥"- that line was cut in the anime due to time constraints but his playful "Aw, ya got me :P" expression was left in, which confirms he's perfectly fine with people knowing he's male. He's not offended at all, he's just "Yeah, that's right".
2. ⁠The toilet scene people like to cite is just a throwaway gag to set up the "crossdresser going into males' toilet" gag you see in anime sometimes.
3. ⁠A line in a manga chapter that wasn't covered in the anime suggests that it's a well known fact that he is male online, so it's safe to assume he outright admits it in his social media profiles. Further confirming the resume thing was just a lighthearted prank on his part.
4. ⁠I don't think I need to point out the obvious in that he calls himself boku (and even ore in a manga line!), though I'm sure someone is going to object to that as well.
5. ⁠It's actually a major point to his character that despite looking selfish at first he actually has a manly gentleman side to him he's not entirely aware of, which is outright pointed out by Miu, his likeliest love interest in the story.

It's not that transgender people don't exist in Japan, but a romantic comedy 4koma is definitely not the medium a Japanese writer is going to use to treat such delicate subjects.

Note- copy and pasted from u/mrshakar