Leukothea is one of three sister goddesses, alongside Fauna and Jordh, tasked with monitoring a human named Kousuke. They detect a dormant "devil's seed" within him, capable of global destruction should it awaken. Despite this threat, the sisters observe positive qualities in Kousuke and choose not to erase him. Leukothea participates in their efforts to safeguard Kousuke and prevent the seed's awakening, using methods described as pleasant interactions for him. An emissary from the Underworld, operating clandestinely as Kousuke's landlady Natsuki, actively opposes the sisters to ensure the devil awakens. Physically, Leukothea stands 1.50 meters tall with yellow/blonde hair and green eyes; her measurements are 76-54-72.