In Undertale, You play as this human named Frisk who falls into a realm full of monsters. You soon realize that you can't get back to the surface world and have to survive in this world of monsters who want you dead. Whenever you fight you use your soul which is depicted as a red heart which allows you to move around anywhere and dodge attacks. Some monsters can change your soul color. For exaple, when your heart turns blue, you can only jump and aren't able to fly anymore. When your soul turns purple you can only move on three purple lines across the screen. Only strong monsters can change your soul color.