Sybil: The Backpack Fairy Vol. 2: Amanite
Nina loves having her fairy friend Sybil around - she's the only person who can see or hear Sybil and the two have formed a special bond. When Sybil unexpectedly disappears, a new fairy named Amanite mysteriously appears to take her place. Amanite tricks Nina into following her to a magical underwater world, where Nina is transformed into a tiny mermaid at the mercy of hungry sea monsters. How will Nina survive without her friend Sybil?