Ryuugajou Nanana no Maizoukin

Japanese Title: 龍ヶ嬢七々々の埋蔵金

English Title: Nanana's Buried Treasure

German Title: Nananas geheimnisvoller Schatz

Chinese Title: 龍孃七七七埋藏的寶藏

Plot
On the "Special Student Zone" artificial island – Nanaejima, the story centers on a beautiful, optimistic, but NEET (not in education, employment or training) girl Nanana Ryūgajō, who was mysteriously killed in her own apartment.

Ten years later, disowned by his father, a boy named Juugo Yama was forced to transfer to a high school on the island. With only a certain amount of money to live on, Juugo chooses a low-rent apartment that happens to be haunted by the ghostly Nanana, who is bound to her former home and unable to move on. Before her death, Nanana collected things from all over the world and hid them throughout the island. Using the mysterious powers of these hidden items, they should be able to find the culprit. As Jugo begins his hunt for her collection, however, he discovers he's not the only person searching. Juugo and members of the school's adventure club join a "treasure hunt royale" on the island for the Nanana Collection.