The story is from manga volume 23.

Hiyori, a small and timid girl, goes to school for the first time in a year since an accident left her hospitalized. Follow her as she struggles with her social skills, tries to make friends and maybe even a little bit more...?

Marina, the most beautiful mermaid in the royal Merman family, is curious about the human world. One day, she sees a handsome prince on a boat. Suddenly, a storm comes up, throwing him into the sea. Marina saves her handsome prince and guides him to the shore. Marina falls deeply in love and desperately wants to become a human. Despite the hazards, she is willing to risk it all for love.

AD 3567. Mankind has lost its once great civilization, now living in the ruins of the aftermath. But even though their lives are threatened by Gardeners — mysterious unmanned mobile weapons (of different ranks) sent from the Moon — brave teenagers have to become Gardener hunters to earn their living. And although human civilization at large lies in ruins due to high-tech maintenance free technology, some parts of the remaining cities still work in limited ways, supporting the humans living there. One fateful day Tokito Aizawa meets Tea, awakening a power older and beyond that of the Gardeners, and inadvertently opening up the door to a possibly much brighter future for the remainder of mankind...