In Ultimate's World of Light opening story, Kirby is the fighter who escapes Galeem's attack.
Galeem, a powerful light-based entity, launches an overwhelming assault that captures or destroys nearly every fighter in the Smash universe. Kirby survives because he escapes aboard his Warp Star, making him the player character at the beginning of World of Light.
The opening intentionally leaves Kirby alone in a devastated world, where he begins rescuing the other fighters and collecting Spirits. As the adventure progresses, players unlock captured characters by winning Spirit Battles and exploring Galeem's enormous map.
A common mix-up is assuming that a major Nintendo hero such as Mario or Link survives. Both are shown among the fighters defeated in the opening sequence. Kirby's survival is also a story device rather than a statement that he is the strongest fighter; director Masahiro Sakurai said the Warp Star provided the practical explanation for his escape.