Shadowcat possesses the ability to pass through solid matter by passing her atoms through the spaces between the atoms of the object through which she is moving in a process called phasing. When Shadowcat is phasing, she is, for all intents ad purposes, intangible. Hence, when attacked, she can shift into a phasing state so as to allow oncoming projectiles to pass through her harmlessly. Since she is unable to breathe while inside an object, she can only continuously phase through solid objects as long as she can hold her breath. She has trained herself to go automatically, by reflex, into a phasing state at any indication of danger. Using her phasing ability, Shadowcat can walk on air. Through practice she has learned to phase other objects along with herself without harm to them, though she must maintain physical contact with the object she phases along herself.

Katherine Pryde lived in the suburbs of Illinois and kept her mutant abilities a secret. When Apocalypse began gathering mutants, Kitty's parents protested and were murdered. Kitty phased out of the house and ran. Kitty was captured by Sabretooth, discovered during a mutant sweep in the ruins of Chicago. Before he could take her back to Apocalypse, the X-Men's Colossus freed her. Kitty became the youngest member of the X-Men. Magneto left her training to Weapon X, who was secretly preparing her to become the X-Men's ghost assassin. Shadowcat was given a pair of wrist gauntlets containing metallic claws similar to those of her mentor and took to her training readily. When Logan and Jean Grey left the X-Men, Kitty lost her mentor and began to question her role on the team. Searching for someone to bond with, Kitty turned to Colossus. The two developed a strong relationship and eventually married.

After a battle in which Colossus froze, the two of them decided to retire from the X-Men. Magneto offered a different solution, asking them to train his next generation of mutant warriors. Kitty wanted to walk away from the X-Men altogether but her husband accepted Magneto's offer. Kitty is the head mistress alongside her husband Colossus who acts as the head master for the kids of Generation Next. During a training day, Magneto made an appearance mid-session and required their services to retrieve a mutant capable of time travel to restore the natural timeline in case the stranger Bishop was correct in asserting that the Age of Apocalypse was never meant to be. The only person to match the description of what Magneto needed was the younger sister of Colossus. Thought to be dead, Illyana Rasputin survived and was being held at the Seattle Core. While their pupils infiltrated the Core from the frontside, teacher and teacheress were able to sneak in from the rear courtesy of Kitty's intangible powers.

They found Illyana but were discovered in the process. Once they had Illyana in their possession, Colossus ordered that the three of them become intangible and floated away to safety, leaving the students behind. Realizing her husband had become obsessed with saving his sister, Kitty started back for her students once they reached the surface. However, Colossus promised to return and help the kids. Piotr would return later claiming that he did everything possible to save his students, although Shadowcat was suspicious and believed her husband may have betrayed his students and left them to die in favor of wishing to reunite with his sister.

Colossus and Shadowcat delivered Illyana to Magneto in the heart of Apocalypse's base of operations, but Colossus became enraged when he discovered Magneto planned to use Illyana to change their timeline to one where his sister was dead. Kitty, believing her husband would never hurt her, stepped in front of him to calm hi down. As he rampaged towards the M'kraan Crystal, she trusted him to stop and so she did not phase. In his insanity, Colossus trampled and killed her.