Mariko Yashida grew up in Japan and was raised by strict parents who held Mariko to incredibly high standards. As a thirteen-year-old girl Mariko realized she was a lesbian. Her parents discovered this and were ashamed. They forced her to hide her true feelings and, as she grew older, Mariko grew rebellious. When Mariko's mutant powers manifested, her parents believed their daughter was cursed. Mariko thought otherwise and used her powers to escape her oppressive family. Mariko flew to the United States and joined the X-Men. She took the name Sunfire and fought valiantly for Professor Xavier. Her parents saw her fighting on television and disowned her.
Shortly thereafter, the Timebroker plucked Sunfire out of her reality and placed her with the Exiles. He informed Sunfire that if she did not help the Exiles succeed in their mission her powers would manifest as uncontrollable radiation.
During her initial missions, Morph helped her adjust to being an Exile and, in the process, developed feelings for Sunfire. Morph eventually revealed his feelings for Mariko, but was rebuffed. Morph still cared deeply for her and agreed to remain friends.
In a reality where the Legacy Virus combined with a techno virus, Sunfire briefly dated Spider-Woman. They formed a strong bond in a short amount of time, but the relationship ended when the team completed their mission and teleported away.
On a subsequent mission, Sunfire was bitten by an Avenger vampire. The Exiles sought to save her by defeating Captain America, whom they assumed to be the head vampire. Regaining consciousness just in time, Sunfire was able to save this reality from the brutal reign of the new vampire king, Union Jack. The dying Union Jack cast a spell on the Exiles as they teleported away and the team was separated.
Nocturne and Sunfire were returned to the Legacy Virus world. Stranded for weeks, Sunfire's relationship with Spider-Woman became very serious. Sunfire was the happiest she had ever been. Eventually, against Sunfire's wishes, the Timebroker reunited the Exiles and Sunfire lost her true love.
Devastated, Sunfire returned to the Exiles. The team discovered they had all been sent to different realities. With no time to rest, the Timebroker sent them on their next mission: the Exiles had to prevent the Thing of the Fantastic Four from destroying New York City. During the battle, the Exiles discovered the Brood had infected Mimic while he was separated from the team. The Brood-controlled Mimic suddenly attacked his teammates. As the team fought him, Mimic collapsed a building and Sunfire, in an attempt to save lives, was crushed and killed. Blink rejoined the Exiles, replacing Sunfire. Later when the Exiles had the opportunity, Morph buried Sunfire's body on Spider-Woman's Earth.