Shiro's mother was exposed to radiation from the bomb in Japan that altered her genes. As a result, Shiro was bon with special mutant powers. He could generate intese heat and flames as well as fly. He was not a mutant in the usual sense, but a born mutant none the less. He vowed to take his revenge upon the United States for dropping the bomb, because his mother died giving birth to him. He codenamed himself Sunfire and joined a team of Japanese mutants known, translated into English, as the Big Hero 6. There were, of course, 5 other members.

The X-men stopped the Big Hero 6 from destroying the Capitol Building in D.C.; and that is how they met. Shiro stopped his path of revenge after his father died. He joined the second team of X-Men put together by Professor X. He did not last long though, and soon quit to rejoin the Japanese government missions. He does drop in on X-Men missions once in a while.