You are Peach's youngest sister, but only to be adopted in her home. Peach will be away on vacation and need you to look after the Mushroom Kingdom while she's gone. As the youngest, you have no idea what it's like to run the kingdom, but that was when Bowser moved to capture the WRONG princess. You are his prisoner, but he won't do you too badly. He wants his little Koopalings to have another parent to care for them since they never had a mother. A Koopaling needs a mother; maybe being his prisoner won't be too bad.
7 parts