Sunfire (Shiro Yoshida) was a former member of the Big Hero Six. He was one-time member of the X-Men and former nationalist who changed his views after the death of his father. Possessing the ability to fly and generate super-heated blasts of plasma, Shiro became one of Japan's most prominent heroes.
Shiro was raised by his uncle Tomo as a child after his mother died from radiation poisoning caused by the atomic bomb dropped on Hiroshima. As a result, he was born a mutant possessing solar radiation abilities. Because of Tomo, Shiro was influenced to hate Western civilization and sought to restore Japan's former glory, despite the fact that his father was an ambassador to the United Nations. Donning a costume representing the rising sun symbol of Japan, he attacked the United States Capitol until the X-Men intervened. However, Sunfire witnessed Tomo killing his father, prompting Sunfire to kill Tomo in a change of attitude. For his actions, he surrendered to the authorities and returned to Japan. Months after the incident, Sunfire was recruited to rescue the original X-Men from Krakoa, the Living Island. However, he denied that his assistance constitutes his becoming a member of the X-Men.
Sunfire would be involved with the X-Man on many occasions; eventually, he would finally join the X-Men after a near-death experience gave him a new outlook on life.
When the Japanese organization known as the Giri began enlisting superhuman individuals to form a national superhero team known as the Big Hero Six, Sunfire was selected as a possible candidate. He initially declines, but later joins the group during a battle with the supervillain Everwraith. One of the members, Hiro Takachiho, would also be acquainted with Sunfire as the mutant was the boy's idol. He would assist the group in protecting Japan, and participate with battles involving X the Unknowable and the Crimson Cowl.
Eventually, Sunfire and the Silver Samurai would depart from Big Hero Six, the former leaving in order to work at the X-Corporation office in Mumbai, India, and the latter gone because he was seemingly slain. With Sunfire gone, his spot on the team was filled by Sunpyre, a young woman with similar solar-based powers. With the two most experienced members of Big Hero Six gone, Hiro became the team's new leader.
- Sunfire, along with Silver Samurai, are the only characters who are removed from the film incarnation of the team, due to the company 20th Century Fox owning the film rights to the X-Men and related characters.