[Spoiler] This is how you get ending D for the mermaid story.

[Vermillion]

On a run I was fortunate enough to have the right stuff to unlock this ending so I thought it would be a good idea to put it down here to help people.
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First you need to acquire the curse 'Insumasu look' (Innsmouth look) from a random event.
Continue to do the case as you would normally.
When confronted with the killer one of the black out options will open up saying reveal 'Insumasu' gills.
When you do this the killer will drop to there knees and say that you are the one he was looking for.
The girl who was mutilated into being a mermaid will jump up and will start to strangle him and he will not fight back.
He will say that 'The ocean will claim you' and your character will pass out before getting the ending but now being terrified of those words and the sound of waves.

[glenn3e]

Ah I just got the gills but my character died while doing the mystery.

[toi chikito]

Thank you tons!