Little Mermaid Ashita He No Tobira (Kodansha KK Bunko)

[Machine Translation] An inspiring story of 15-year-old Ariel's adventures and growth! For Ariel, the Princess of the Sea, the Coral Moon at age 15 is a special day. It is the day that a part of the Sea Kingdom becomes her own territorial waters and she is appointed as the guardian of it. Her sisters, who had been separated since her mother's death, gather for the first time in many years to attend the inauguration of Ariel's guardian. However, one of the sisters, Mara, has gone missing, leaving behind a mysterious message! Ariel, together with Flander and her other sisters, sets out on a dangerous journey into the unknown sea in search of Mara...? From middle elementary school age.

15歳のアリエルの冒険と成長を描く感動ストーリー!海の王女アリエルにとって、15歳のコーラル・ムーンは特別な日。海の王国の一部分が自らの領海となり、その守護者として任命される日なのです。母の死後バラバラになっていた姉たちも、アリエルの守護者の就任式に参列するために久しぶりに集まります。ところが姉の一人マラが、謎のメッセージを残して行方不明に!マラを探しに、アリエルはフランダーやほかの姉たちとともに危険な未知の海へ旅立ちますが―!?小学校中学年から。