IF YOU DO NOT WANT TO BE SPOILED, DO NOT READ THIS:
Baymax is basically a gift his late brother gave him. Robot or not, Hiro loves Baymax and even went to rebuild him to stay with it. Baymax became something he’d care as much as his own brother. This drawing isn’t to say that Baymax IS Tadashi, but to say that it is the only thing that can confort Hiro almost as well as Tadashi did.