Betty Valentine, princess of the demonic world, possesses a devilish nature paired with an angelic face and a fundamentally kind heart. With striking blond hair and notable beauty, including prominent breasts, she arrives in the human world on a critical mission: to defeat the demon Lutan, also known as Routain.

During a rainstorm, she encounters Tanpei Kimogawa, an earthbound gangster focused on avenging his parents. Found naked and wounded on the ground, she immediately captures Tanpei's attention. After she later rescues him from bandits, they rapidly develop romantic feelings and move in together, cohabiting in a Tokyo high-rise apartment.

Their relationship faces challenges from Betty's demonic family. Her skeletal grandmother sends nubile nymphs to test Tanpei's suitability as a husband. Betty also contends with frequent, inconvenient intrusions into their daily life by her witch and demon relatives.

Beyond her partnership with Tanpei, Betty is later depicted as a mother to a daughter. Her royal demonic background and her core duty to defeat Lutan remain consistent throughout her story.