I think you're pretty interested in this Manga/Anime.
I will tell you a few things about it.
Admin's note: Of course it's all about headcanons...and Himaruya didn't agree. But that's how I imagine it. Please be respectful ^^
(It can be happen that...
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I like Prussia Headcanons too much.
Anyway...
Prussia absolutely hates it to be referred as a country which was on Germany's side during World War 2. He doesn't care if it's his brother, when something triggers him, it's that. No matter who it is, in which situation, when you tell him that he was for the Third Reich, he will instantly hate you. Prussia will eventually slap you and just go away and don't talk with you for days.
He also hates it to speak with someone about it, due to what his brother did...
Do not mention it in front of him. He can be really, really triggered. And nobody wants a triggered Prussia.
