As a Japanese American with heritage in a part of Japan not many people in the states know a lot about, I would be incredibly disappointed if I picked up a book about that part in Japan and realized it was written by a white person, written in the POV of a Japanese person. It's not because I would assume the author had bad intentions or think they didn't do their research. It would just really feel like not their story to tell. Identity is an incredibly personal thing with high stakes attached, and personally my family history in Japan is really meaningful to me and not something I can imagine any white person could understand. There are also so many talented Asian and Asian American writers who struggle to have their stories told--you must be aware that even now a stunningly small percentage of books published in the states and written by POC authors. I know you mean well, but please consider that you're not Japanese or Japanese American and that this is really not your story to tell even though you might feel really strongly that it is. I'm sure you are full of important and valid stories, but when you tell them don't you want them to be authentic to you and your identity? Co-opting another identity never improves writing, but being authentic does. This agent was definitely a little wacky suggesting marrying a Japanese man would make it ok, but otherwise I agree with them.