The very interesting question of who! That's a mystery and even a community as smart and big as Bitcointalk fail to pinpoint him out accurately (together with evidence) in short. Why did he hide his identity is something easier to answer though but instead of that, I'm actually wondering how he's cashing out his billions 
So interesting that even the people that is not a member of this forum is also trying to figure who really is Satoshi Nakamoto. There might be some things that can track his place, stikl he is ready with it that it can't be tracked by simple means. And far as I know, he has no billions because he left bitcoin.