Here is the mermaid quest.
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Fot the hints:
Sorry i do not know them just based on the hint numbers, would need the hint text as well to know.
One is most likley for the one episode you have still open i assume?
Rabbit currently is the only party member with a transformation. So you not missing them.
That are is a secret area and 100% completely optional to the game so i wont spoil the answer. But for a hint: With a bit of "luck" a npc in the snow town might give you a nudge towords the solution