Mermaid falls in love with merchant.
However, in this world, fishmen are feared existences. For example, the first time little sister elf saw Geto, she screamed, hid then prayed to god to protect her.
So a relationship was impossible.
Mermaid stole a fishmen treasure and used it to change her appearance into a human.
It worked, but ironically, it took the love the mermaid and the merchant had for each other as the price.
Hence, Geto got extorted.
Souya went to negotiate with merchant, saw the treasure sitting on the desk, had a bad feeling, and shot it without thinking.
Spell broke, mermaid returned to normal, but their love for each other returned.
They couldn't get married officially, but the merchant and the mermaid said screw it and lived as man and wife. Geto forgave them and gave his blessing.
Happy end.