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 crime lord 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.