Mermaidyn evolves into Mermadonna when fused with a Mermaid's Jewel. She is also one of the Yo-kai required to unlock Spoilerina.
Mermaidyn can also evolve into Scaremaiden when fused with a Wicked Parts.
Mermaidyn are fair-skinned mermaids with sapphire eyes and black hair tied into a chigo mage (Japanese ringed hairstyle). Sidelocks hang past their pointy ears and has dot eyebrows. They wear seashells to cover their chest and a pale blue hagoromo. The fish tail is red with a white fin fading to pink, with a matching one on top of their heads.
Mermaidyn are elusive but generally patient creatures who might sometimes be fished up by accident. The Mermaidyn 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.
Mermaidyn likes to spend a lot of time improving her appearance, such as with Yoga, facial treatments, and even bedazzling her tail.
For some odd reason, Nate is only ever able to fish up one particular Mermaidyn, rather than normal fish, no matter where he goes fishing. Lakes, streams, bathtubs, buckets, even puddles in the road all turn up Mermaidyn. Mermaidyn 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 Mermaidyn is, though he has invoked their relationship once during Halloween, where she exclaims that Nate should simply have used his medal.
Mermaidyn is automatically befriended through the main story.
Mermaidyn can also be found at Gourd Pond in Old Springdale.
Receiving food (favourite): "Mmm, revitalizing!"
Receiving food (normal): "Phew."
Receiving food (disliked):
Freed from Crank-a-kai:
In EP073, Nate first meets a Mermaidyn while trying to fish up something rare to impress his friends, catching her on his hook. Mermaidyn 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 Mermaidyn again. To the horror of both involved, Nate finds that, no matter where he fishes, all he catches is this one Mermaidyn, his attempts at not catching Mermaidyn culminating with dropping his fishing lure into a puddle in the street. Upon being caught this time, Mermaidyn 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 Mermaidyn.
During EP093, Instead of summoning her via the medal in the Yo-kai Watch, 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 Faux 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.
"Ningyo", along with being the Japanese word for mermaid, is also a pun with "Kingyo", the Japanese word for Goldfish, to align with the yokai having a goldfish tail. Mermaidyn is a portmanteau of "mermaid" and a corruption of "maiden".
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 recap time, she has an unseen friend who can petrify by being looked at, similarly to a gorgon, possibly (in particular) the Greek monster Medusa Gorgon.
Her Sultimate'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.