To get an alternate costume for Yoko, play as Yoko complete the "Outbreak" stage under the normal difficulty setting. Once done, go to the "Collection" option at the main menu and choose "Costumes". Yoko's costume will cost 8,000 points. Once it is unlocked, you can play as Yoko before she changed into the person she is. Note: Yoko is the only character who has an alternate costume.