Warning: SPOILERS for Deadpool 2 are ahead!
In the first Deadpool movie, Wade Wilson made a crack to Negasonic Teenage Warhead about how he only ever saw her and Colossus at the X-Mansion, speculating that 20th Century Fox couldn't afford to include another X-Man. This was slightly rectified in Deadpool 2, and not just because we briefly saw the Dark Phoenix-era X-Men as Wade was moping around and the Origins-era Wolverine during the movie's mid-credits scene. Accompanying Colossus and Negsonic Teenage Warhead in the sequel was Yukio, the latter's girlfriend who will arguably best be remembered for the chipper way she said, "Hi, Wade!" Like most of Deadpool 2's other characters, Yukio hails from the Marvel Comics universe, but the version we just saw on the big screen differed significantly from her comic book counterpart.
Introduced in 1982's Wolverine #1, and created by Chris Claremont and Frank Miller, Yukio is an adult character who was a thief and ronin (a masterless samurai) who was serving the villainous Shinegen Yashida when Wolverine first crossed paths with her. Although Yahsida, who was also the father of Logan's then-girlfriend Mariko, originally tasked her with killing Logan, she developed feelings for him and later helped him take down Yashida. Yukio doesn't boast any mutant abilities, but her fighting and weapons skills make her a formidable opponent. During Logan and Mariko's wedding, Yukio worked with Storm to stop Viper and Silver Samurai from killing Mariko. Unfortunately, Wolverine's wife died by by his hands later after she was poisoned by a different assassin, asking her husband to make her death quick rather than suffer through the excruciating pain.
Wolverine subsequently entrusted the care of his adopted daughter, Amiko, to Yukio after discovering that her original foster family was abusing her, and Yukio trained the girl in martial arts. When Wolverine's arch-nemesis, Sabretooth, was targeting Logan's friends and family, he sent Omega Red and Lady Deathstrike to go after Yukio and Amiko. Both survived, but Yukio was crippled during the attack, and while there was a period where she was fully healed, she returned to being confined to a wheelchair following an encounter with Hellverine, a.k.a. Wolverine possessed by a demon.
When actress Shiolo Kutsuna was first seen in one of the Deadpool 2 trailers swinging around an electrified chain weapon, it was speculated that she was playing Surge, a member of the New X-Men. But Deadpool 2 co-writer Rhett Reese clarified to Comic Book Resources that Kutsuna's character was just an altered version of Yukio, saying:
She's a super-minor character. The character's name is actually Yukio, an assassin --- but who's an adult assassin, so we just aged her down. She's such a minor character, we thought we could pretty much get away with anything.
It's important to mention that this isn't the first time we've seen a version of Yukio in the X-Men film franchise. She was previously portrayed by Rila Fukushima in 2013's The Wolverine, but this Yukio more closely resembled the version from the printed page, although she was also a mutant who experienced visions of the future. When we last saw Fukushima's Yukio, she was accompanying Logan as his bodyguard, but she was nowhere to be seen in the movie's post-credits scene, and X-Men: Days of Future Past rewrote this universe's timeline the following year. Between the X-Men films never being strong sticklers for continuity and the Deadpool movies almost feeling entirely separate from the franchise's other entries, Shiolo Kutsuna's Yukio should basically just be considered an all-new character. While Rhett Reese expressed interest in exploring Yukio more, it remains to be seen if she'll be brought back for Deadpool 3 or X-Force.