Little is known about the early life of the adventurous Yukio save that she worked alongside the mutant thief and future X-Man Gambit, who betrayed her during a caper in England. At some point in her life she visited the Valley of Mercy and Wrath, supposed homeland of the ninja clan called the Hand.
As an assassin for the Japanese crimelord Lord Shingen, leader of Clan Yashida, Yukio became part of Shingen's campaign against the mutant hero Wolverine. She drugged Wolverine and brought him before Shingen, who proceeded to shame him in battle. Later, Yukio manipulated Wolverine into helping her kill Shingen's rival, Katsuyori. Attracted to Wolverine, Yukio chafed at his love for Shingen's daughter Mariko but nevertheless saved Mariko's life when her abusive husband threatened her.
Following Shingen's death, Yukio aided Mariko and Wolverine against attempts by the criminal Viper and her bodyguard, the Silver Samurai, to take over Clan Yashida. During this time, Yukio began a lasting friendship with one of Wolverine's teammates in the X-Men, the weather-manipulating Storm. Yukio then teamed up with Wolverine when he sought to rescue his teammate Kitty Pryde from his former teacher, the demon ninja Ogun. Yukio saved Mariko's life once more, this time from an assassin in the employ of Ogun's ally Shigamatsu.
It is unclear when Yukio joined the Mutant Underground, a covert mutant rights activist organization, but her activities in this area led her to be herself targeted by the techno-organic alien collective the Phalanx. Fleeing to New York, Yukio allied with the X-Men and helped defeat the aliens.
Wolverine ultimately entrusted the care of his foster daughter Amiko to Yukio, who promised to raise her as if she were her own. Soon after, she helped the mutant government operatives called X-Factor when a mission brought them to Tokyo, joking that American mutants venturing to Japan would be lost without her aid. Later, Yukio and Amiko were captured by a faction of the Hand but were rescued by Wolverine; however, Amiko remained partially within the Hand's thrall. When Yukio aided the ninja Elektra against the Hand, accompanying her to the Valley of Mercy and Wrath, Amiko warned the assassins of her activities. Yukio and Elektra nevertheless escaped their encounter unharmed, although it remains unclear if the Hand maintains any hold over Amiko's mind. Yukio and Amiko were again abducted, this time by the criminal Gorn Kaishek to force Wolverine to help him defeat Gorn's brother Haan, who had taken over Clan Yashida. When both Kaisheks were betrayed by their sister Kia, Yukio managed to escape with Amiko, while Wolverine and the Silver Samurai dealt with the remaining Kaishek forces.
Yukio and Amiko were next targeted by the cyborg Lady Deathstrike and the Russian super-soldier Omega Red, two old foes of Wolverine's who had been hired by his nemesis, the feral mutant Sabretooth. Yukio was crippled by Deathstrike, who then carved a message to Wolverine into her back. When Wolverine visited her in the hospital, Yukio asked him to kill her, as she was unwilling to live as a paraplegic, but he chose not to do so.
Yukio slowly recovered and was able to aid Storm during her investigation of an underground mutant fighting arena in Tokyo. Despite her setbacks, she remains a life-loving adventurer, eager to face any challenge alongside her friends or alone.
