Kumi Yamakado

As one of the seven mermaid princesses, Lucia must travel to the human world to protect the mermaid kingdoms.

Humanity makes much progress through a combination of science and magic, until a large war breaks out. Many years later, the magical city of Sosarium (constructed on the Moon) sends an agent down to Earth to make sure history does not repeat itself as civilisation starts to rebuild itself. That agent is known as an Angel.