UFO Ultramaiden Valkyrie

A princess from outer space accidentally crashes on Earth, where Kazuto desperately tries to maintain the public bath of his grandfather. Due to circumstances, Kazuto receives part of princess Valkyrie's soul which forces her to stay there with him. But that is not the only problem... because her soul lost strength, the 18-year-old princess transforms both mentally and physically into a 8-year-old little kid.