What we do know, is that Hiro's kind-hearted older brother Tadashi, is dead; he is the one who left Hiro with Baymax. It's incredibly unfortunate, because he's considered incredibly gifted, not to mention a super rad older brother.
What do you think happened to him? Most people think it was Yokai; if so, what do you think he has to do with Yokai that would make it/him kill Tadashi?
just leavin this out here for ya brains to contemplate