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

There was once a soldier who defended the Earth valiantly against the Invaders until he was defeated. Half of his body was covered in lethal acid before he spoke his final word. Who was this word meant for? His family? A friend? Or perhaps, a lover? No...he must have been speaking to the soldiers entrusted with the fate of the Earth. But this was in another world, on a round planet. In this square Earth, he has returned to the battlefield, the majority of his body now mechanized using the latest technology. And in his hand? An Acid Gun...

More details on the official game page of The Legendary Mr. Y, Reborn as "Mecha Yuki" After His Acid Incident!? ...