Wakasagihime is a mermaid who lives in fresh water. She is a peaceful youkai who spends her days singing and picking up stones. She once went on a rampage in the Misty Lake during an incident, but she is living a quiet life now.
Apparently, her power is stronger in the water, but most battles take place in the air.
Wakasagi, or Japanese smelt, is a fish in the order Osmeriformes of the family Osmeridae. It is a bony fish that lives in cold water. There are two types: anadromous (fish that migrate upstream) and resident (fish that spend their whole lives in fresh water). They are often used in food. Lake Suwa is famous for wakasagi fishing.
Japan's mermaid legends began in the Nihon Shoki, or The Chronicles of Japan. There is a story of Prince Shoutoku meeting with a mermaid in Omi Province and providing it with a memorial. While there are many bizarre tales that speak of mermaids as something horrific, there are legends that speak of them as good omens as well. Mermaid mummies are sometimes used to pray for longevity and good health.
The legend of Yaobikuni, or the eight-hundred-year-old nun, is about a nun who ate the flesh of a mermaid and gained perpetual youth and longevity. Finally, she wound up in Wakasa, where she ended her life.