This allows you to use a special character and that character's weapon.

Aya, the calm and composed swordswoman from the zombie-infested world of "OneeChanbara," has joined the fight, wearing not only her trademark 10-gallon hat and bikini, but also with a katana as her weapon. Usually focused on exterminating zombies, she has decided to break from her normal routine and use her sword-wielding expertise to help save the Earth. She may appear less "herself" here compared to her appearance in the "round" world, but that's to be expected in a world without curves. For her SP Move, she unleashes her powerful "Xtasy Combination," which feels as good as the name suggests when it hits.

More details on the official game page of Aya, the Calm Big Sister Swordswoman from "OneeChanbara" ...