Honestly, if the characters were Japanese people in an English-speaking country and they weren't fluent, I'd get a bit of Engrish being thrown in, but that's not the case here. This is set in Japan, things are going to either be written in Japanese or in English. The former preferably just in the case of proper nouns. I'm fine with hero names being Japanese, but if it's just going to be English words written as if they're being said with a Japanese accent, it should be written in English.