Hiro might have to fight his own brother, Tadashi, in the upcoming Big Hero 7. Tadashi to come back as the uncanny avenger, 'Sunfire'.

In Big Hero 6, fans' hearts broke when Tadashi died after being trapped in the burning building filled with different inventions. Many thought, everything about Tadashi stopped there, but, that is apparently not true, as Tadashi is said to come back for the movie's sequel, Big Hero 7.

In Big Hero 7, however, Tadashi will not be Hiro's good big brother, as he will be possessed by a villain.

He will come back as Sunfire, the 'uncanny avenger' as described in the comics. Unfortunate as it may seem, Hiro and Tadashi will reportedly come face to face and fight against each other, although many still speculate that Disney might find a way to turn him back to being good.

In the comic books, Sunfire has been mentioned as a member of X-Men. He is a Japanese mutant who has the power of releasing flames by absorbing heat.

The confirmation reportedly came from Mr. Stan Lee himself, as he mentioned the movies to be released by Marvel which included the Big Hero 6 sequel. During an event promoting the Avengers razor series, Lee reportedly talked about the upcoming movies saying,

"After Ant-Man, we're going to start playing around with Doctor Strange, the Black Panther, the Inhumans, and then we have to come back for Guardians of the Galaxy 2, Big Hero 6, The Avengers, Captain America... I don't even know how they decide what to do next, because every one of them is so exciting and the public is waiting for them" 

Big Hero 6's income also seems to bode well for the sequel as it reportedly grossed $651 million in worldwide sales.

No release date has been announced for Big Hero 7 yet, although it might be released in late 2017 or early 2018 after 'Guardians of the Galaxy 2'.