Oh, boy! As an American consumer, I'm even more perplexed what the hell they are.
Like 15 years ago, Rakuten seemed to be a normal ecommerce site. I think they bought buy.com or something to get a foothold in the US market. Then they pivoted to being some sort of cashback referral service.
I'm not really sure why that would lead customers to think "yeah, I want cloud storage from the people who made a weird janky digital simulation of the Piggly Wiggly Value Club Card!"
(AWS made it work because they could say "we have the infrastructure to host one of the busiest sites on earth, it's good enough for you", but Rakuten does not have that credibility in the US)
They do a lot of things, mostly related to online retailing in Japan. They bought Ebates, presumably to get all the shopping data they collect, it wasn't really a pivot. (Also they'd been doing financial-ish stuff like rewards programs before apparently. I went and read their Wikipedia article when I saw this post.)