According to Rakuten Inc's family tree, as listed on Hoovers.com and viewed by Ethical Consumer in August 2018, the company had multiple subsdiaries in jurisdictions considered by Ethical Consumer to be tax havens at the time of writing. Of these, two were holding companies which were considered high risk subsidiaries for likely use of tax avoidance strategies:
Play Holdings Ltd (Jersey): holding company
Rakuten Europe Sarl (Luxembourg): described as "providing computer processing and data preparation services" but also acting as the parent holding company for a number of other subsidiaries in Luxembourg and elsewhere (Germany, Spain, UK, Ireland, USA).
The company published no country-by-country financial information.
Given that Rakuten Inc had two high risk subsdiaries and the company published no country-by-country financial information, it received Ethical Consumer's worst rating for likely use of tax avoidance strategies.

Reference:

Generic Hoovers ref 2018 (2018)