It's potentially a bit inconsistent, too. While Mirako and Tenma are solidly Japanese names, Lee is a Korean name (unless we assume it's the English-language name instead). To me, it seems like Mirako, Lee, and Tenma are all actually multiple generations down from the first generation born in . . . wherever the story actually takes place. They are Japanese by ethnicity (maybe?), but they are American/English/whatever by culture. I'm not sure how rigid traditions would remain once you're several generations into a completely different culture. Most of my Japanese, Korean, Chinese, Filipino, etc., friends didn't adhere to the traditions of their ancestors because they were born in America and didn't see a need to continue them.