Baymax is a robotic synthformer created by the young genius Hiro Takachiho when Hiro lost his father Tomeo Takachiho. Baymax was seemingly built to be just a bodyguard but soon helped fill the void left by his father’s death, certainly helped by the fact that Hiro installed many of his father’s memories and emotions into Baymax’s core programming. When Hiro was drafted into the Japanese hero group, Big Hero 6, Baymax came along for the ride. Hiro reluctantly joined to prevent Everwraith from slaughtering millions in downtown Tokyo. During the battle, Big Hero 6 was joined by Sunfire, Japan’s premiere super hero and a mutant with the ability to super-heat matter into plasma, who was instrumental in the Everwraith’s defeat.Soon after, Big Hero 6 moved their headquarters from the Giri Office Building to Japan’s Cool World Amusement Park, where they were attacked by X the Unknowable, a monster born from a child’s drawings that could transform its atomic structure into any form and shape. With the help of Alpha Flight, Big Hero 6 destroyed X the Unknowable in the fires of Mount Fuji. Afterwards, Big Hero 6 continued to protect Japan from all threats, such as when the team rescued Tokyo residents from a freak blizzard caused by the Crimson Cowl (Justine Hammer) and her Masters of Evil.The members of Big Hero 6 fell victim to a mind-control device implanted within Baymax. Traveling to Canada, the mind-controlled super heroes attacked a new incarnation of Alpha Flight at a national park. After a brief battle, the mind-control device was short-circuited and the two teams parted as friends. Big Hero 6 returned to Japan to seek out the parties responsible for their mind-control.
Got this from marvel wiki