And the yokai in this chapter is "futakuchi onna"
https://yokai.fandom.com/wiki/Futakuchi-Onna
Yikes
"In another story, the extra mouth is formed when a stingy woman is accidentally hit in the head by her husband's axe while he is chopping wood, and the wound never heals. Other stories have the woman as a mother who lets her stepchild die of starvation while keeping her own offspring well fed; presumably, the spirit of the neglected child lodges itself in the stepmother's or the surviving daughter's body to exact revenge.,"
So if we use Japanese's mermaid folklore, the ending will be our MC eating the fish girl instead and continue to suffer for eternity, or at least 800 years to muster the courage to end her own life.
