The Extras are skins.
The first skin is unlocked by finding the arm of the giant neko statue and bringing it to her in the secound level
The secound one is unlocked if you finish the game without dieing and without killing anyone
The third one is unlocked if you beat the 10th vr mission level wich you can find int the room next to the bedroom in the home of the main character (the 10th level is only unlocked if you have beaten all 3 bosses)