Mermaid

Mermaid (人魚) is a high-ranking supernatural in the Jibaku Shounen Hanako-kun series. She is able to manifest in Kamome Academy while living underwater with other fish, although it is unclear whether that place is bound to the school.

Appearance
She appears to be a giant fish with humanoid features, such as long messy hair decorated with barnacles and sharp teeth. Her fins are long and end in claws.

Personality
Because of her high ranking as a supernatural and old age, she thinks highly of herself. She speaks in a regal tone and regards the School Wonders as mere apparitions. This is shown in her encounter with Hanako, a supernatural who was born in only the last recent decades. She is quick to dismiss him, caring only about retrieving her new servant.

It is later revealed that she is actually very kind and caring towards her subordinates.

Mermaid Scale (Matchmaking Charm)
Her scales have the power to curse individuals who swallowed them to become the Mermaid's servants. The servants will be transformed into a fish and are collected by the Mermaid herself personally.

When the scales are shared between two people, the effects of the curse is lessened. The subject wouldn't turn into a fish fully, but just become fish-like. At the same time, these individuals will be bound by a very powerful bond, and the scales are referred to as a "Matchmaking Charm" as a result.

By sharing the scales with Hanako, Nene was able to gain the ability to interact with him as if he was a normal human. Hanako mentions keeping the effects of the curse to a minimum through his powers, implying the individuals would have kept more of their fish-like appearance under normal circumstances.

Great Mermaid's Life Blood
Her blood has the powers to separate the matchmaking bond between two people made with her scales while also bonding the drinker of the blood to her.

Trivia

 * The Mermaid is probably based on the Ningyo, a fish-like creature from Japanese folklore.