The short answer
Lead with Rakuten when brand storefront control and loyal repeat buyers matter; prioritize Amazon Japan with FBA when convenience and lean operations matter most.
For brands entering Japan, the marketplace decision shapes both economics and how much brand you can build. Rakuten Ichiba and Amazon Japan reward very different buying behaviors.
Rakuten runs on a merchant and points economy that rewards loyalty and lets you build a richer branded storefront. It suits brand-building entry but requires more merchant operation.
Amazon Japan rewards convenience and Prime-driven purchasing, with standardized listings and FBA handling storage, shipping, and returns. It lowers operational overhead but offers less brand control.
Either way, localized product data and copy — not translation — materially affect conversion. Many brands eventually run both, but should localize listings, pricing, and ads per platform rather than duplicating.