Action: During a Water conflict – put this character into play from your hand in the conflict. At the end of the conflict, shuffle it into your conflict deck.
Ok, so when you use this card, you trigger its action from your hand. As I understand it, it creates a delayed effect that activated when the conflict resolved, shuffling it into your deck.
Now, what happens if the Feral Ningyo ends up in your discard pile before the conflict ends? Be it because your opponent uses a card like Noble Sacrifice or Shoju, or because you sacrifice it.
Is the delayed effect still active, so at the end of the conflict Ningyo still goes to your conflict deck, even from the discard pile? Much like Display of Power, that creates a Delayed effect, then goes to the discard, and the effect triggers some steps latter.