Why would you ever take on such a case? A matter were testimony stands against testimony? That would totally ruin your wonderful 99% conviction rate [0]. If a victim comes to you to report something like that, you better get her out asap by whatever means necessary. Just make sure you're not become part of the crime stats yourself... though that should be hard for you thanks to the necessary inside knowledge & contacts.
Anecdata: a female friend of mine was beaten up by her boyfriend (who was a pimp at a "soapy") and sent me pics of the bruises. I told her to go to the police. She said they would do nothing, something about her dad having a bad reputation (I didn't fully understand her Japanese text messages). I said go anyway and at least try, she's got clear evidence.
She did. They did nothing. And I've got several other similar stories from other women I've known. There is a LOT of unreported domestic violence and sexual assault in Japan.
The conviction rate is high because prosecutors have discretion over which cases to take to court, and there is no jury. The outcome of a bench trial is often predictable, and prosecutors will only take to trial cases they believe will succeed.
A large proportion of prosecutions in Japan rely on confessions, often obtained by duress. Even if only a small number of perpetrators (or innocent suspects) confess, taking only those cases to trail keeps the conviction rate high. The rate at which crimes are solved is somewhat lower -- less than 30% according to the article.
[0] https://en.wikipedia.org/wiki/Criminal_justice_system_of_Jap...