First you need to know the this great movie comes from an old Marvel comic, (if you din't notice the Stan Lee cameo, well that's why he's in there!) and the name of this comic it's "Sunfire and The Big Hero 6" from back in 1998 and he was the only japanese hero back then so Marvel came up with a more intense Origin Story for the Big Hero 6.
Ok! well Sunfire from the X Men got he's first series on Marvel, and was a bit tragic because he was dead on the first book and the story follows a 13 year old boy named Hiro Takachiho, and was a huge fan of our fellow mutan friend and also happen to be an incredivly genius, among He's creations there was this robot call Bay Max who act like he's friend, nanny and protector .
Then from an Unfrendly face, "Silver Samurai" wicht he's no longer a villan by that time and explains to Hiro that Japan needs a Defending Heroic Group of their own like the Avengers, Fantastic Fours or Alpha Flight. The "Giri Industrial Corporatin" was founding the creation of such a team witch was to be name "Big Hero 6", the Silver Samurai got the task to from this group and wanted Hiro and BayMax on the Team, Hiro refuses the idea. Later Hiro located he's hero Sunfire and discover that he was dying from a rare illnesses.
Convinced later Hiro and Sunfire by the two newest members Go Go Tomago who can control huge amont of energy and Honey Lemon who carry a purse that can contain any posible gadget for a necessary situation, wicht complete the now know "Big Hero 6"
Sunfire was taken away by Silver Samurai to help him, and some crazy old dude made of all the spirits from the atomic attack from 1940 wanted Sunfire to recreate the atomic event to make Japan more powerfull, Sunfire and Big Hero 6 defetaed the enemy at a huge cost, Sunfire's life. Later Hiro finds out Sunfire fake his own dead again and promise to keep hes Secret.
Then the series change. But on 2008 Big Hero 6 got renewed with a diferent origin story, when Disney decide to make the renewed version of Big Hero 6 into a movie they decide to keep little fragments from the original comic book to make the story a bit dramatic, making Tadashi last scene in a fiery explosion, and I know it's a bit grim but still an easter egg and that's my theory