The supervisor is Nobuaki Kobayashi at Nagashima Ohno & Tsunematsu in Tokyo.
If you have evidence of fraud or unauthorized transactions, I'd suggest putting it into a clearly written document, have it translated to Japanese, and sending the document, in both English and Japanese, with a suitable cover letter, to him at the firm's office in Tokyo.
Their fax number is +81-3-5213-7800. That will put it into the record of the case. It's entirely appropriate to contact the bankruptcy supervisor if irregular activity is discovered.
(One way to get a semi-decent translation of your own writing is to run what you've written through Google Translate to Japanese and back to English. Rewrite your text until it's clear after double translation. Then you have some confidence that it will be understood at the receiving end. Write in English with a very formal tone, and hope that makes it through the translator.)
(You can send faxes via PDF-to-fax services if necessary.)