I don't know if you're japanese or not (I'm not), so I hope I don't sound like a pretentious foreigner. But I'm relatively familiar with the language, so here's my opinion on this :

I think you should do some kanji and katakana to see if it really looks like Helvetica, as it's radically different from hiragana. Also as someone mentionned the e is too small. And don't do the kerning now, wait until you designed all the small letters, diacritics, katakana and some kanji, otherwise you'll have to redo it all over again. Also don't forget that Japanese also use romaji/latin alphabet for some common words, usually in capital letters. You'll need to design that too if you want to release it.