Satoshi may be one person or may be just the name behind a group of people who all created the block chain technology and Bitcoin. So far there have been a couple of people who claimed to be Satoshi, but not even one of them managed to prove it. About why he is not showing his true identity there are many theories - imagine you were that person with such a big stash of bitcoins in your possession? Are you gonna show yourself? Another theory is that he is already dead and that's why those coins from the early blocks are not moving at all. Another one is that he went nuts and lost his private keys and even if he shows himself no one will believe him, because he can't prove it. And there are many more theories, but you get my point...