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

Saki, the crazed swordswoman from the zombie-infested world of "OneeChanbara," is here and ready to fight using her super long katana and acrobatics. Like her big sister Aya, she usually spends her time exterminating zombies, but she couldn't sit idly by while the Earth was in pieces. 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 uses her family's Baneful blood to temporarily boost her attack power and become invincible. Now, THIS is truly Baneful power.

More details on the official game page of Saki, the Crazed Little Sister Swordswoman from "OneeChanbara" ...