Mermaidyn

is a Water-attribute D-ranked Classic Yo-kai of the Slippery tribe.

In the Yo-kai Dictionary, it states, "A Yo-kai who has the upper body of a human and the lower body of a fish. She sings with a very beautiful voice. She has a lot of interest in people of the land since she is always in the sea."

She evolves into Yao Bikuni when fused with a Mermaid's Jewel. She is also one of the Yo-kai required to unlock Netaballerina.

Appearance
A fair-skinned mermaid with sapphire eyes and black hair tied into a chigo mage (Japanese ringed hairstyle). Sidelocks hang past her pointy ears and she has dot eyebrows. She wears seashells to cover her chest and a pale blue hagoromo. Her fish tail is red with a white fin fading to pink, with a matching one on top of her head.

Personality
Ningyo are elusive but generally patient creatures who might sometimes be fished up by accident. The Ningyo that appears in the anime, however, seems fated to be the only thing Nate can ever fish up no matter what, and as a result is far more foul-tempered around him. She also refuses to go along with the mischief the other Classic Yo-kai get up to on Halloween, but winds up dragged along, proclaiming it as a bad idea, being proven right as the Yo-kai are baited into revealing their monstrous true natures by hecklers.

Ningyo likes to spend a lot of time improving her appearance, such as with Yoga, facial treatments, and even bedazzling her tail.

Relationships
For some odd reason, Nate is only ever able to fish up one particular Ningyo, rather than normal fish, no matter where he goes fishing. Lakes, streams, bathtubs, buckets, even puddles in the road all turn up Ningyo. Ningyo was patient with Nate at first, but became irritated the more Nate unintentionally fished her up, finally giving Nate her medal on the agreement that he never fish ever again. Nate is about as thrilled with their fated arrangement as Ningyo is, though he has invoked their relationship once during Halloween, where she exclaims that Nate should simply have used his medal.

History
Nate first meets Ningyo while trying to fish up something rare to impress his friends, catching her on his hook. Ningyo is not the sort of rare game he had in mind, however, and releases her after a quick apology. He tries a different spot, only to fish up Ningyo again. To the horror of both involved, Nate finds that, no matter where he fishes, all he catches is this one Ningyo, his attempts at not catching Ningyo culminating with dropping his fishing lure into a puddle in the street. Upon being caught this time, Ningyo agrees to hand over her medal, demanding that Nate never goes fishing again. Nate breaks down and cries, as he realizes he's cursed to only ever catch Ningyo.

Nate invokes their odd relationship during Halloween when he fishes her out of a bucket, much to her dismay. She points out that the mischief the other Classic Yo-kai have planned involving blending in with the costumed humans is a bad idea, and even Kappa's charm cannot calm her down. She winds up dragged along in a kiddie pool and tries to keep the other Classic Yo-kai from being baited into revealing their true nature, but to no avail.

Trivia

 * In recap time, she has an unseen friend who can petrify by being looked at, similar to Medusa.
 * Her Soultimate's Japanese name is a reference to Nagisa no Haikara Ningyo (渚のはいから人魚, "Stylish Mermaid Of The Beach"), a 1984 song by Koizumi Kyōko.

Origin
Ningyo is based on the youkai of the same name, sometimes called the "Japanese mermaid". Originally portrayed as humanoid fish with a row of tiny teeth and golden scales (or rather the same color as a goldfish, called kingyo in Japanese), with time it became increasingly common to see them as unearthly beautiful half-fish women, similar to Western mermaids. They are inhabitants of Ryūgū-jō, the undersea castle of the marine dragon god Watatsumi.

Their flesh is said to be incredibly tasty and delicate, and to grant near-immortality when eaten; but at the same time those who kill a ningyo will be cursed with great misfortune for harming such a pure, beautiful creature. Seasoned fishermen will take care to immediately release a ningyo if she's accidentally caught in their nets, lest their incur the wrath of Watatsumi.

In other languages

 * Japanese: にんぎょ Ningyo
 * Korean: 인어 Ineo