Lantern

Lantern is a mermaid.
That's the long and short of it.

Lantern, however, isn't your Princess Ariel, copy-paste human torso fish tail, type of mermaid. And even amongst that breed she's a bit of a marvel. Lantern comes from a more archaic type of mermaid, based largely on deep sea creatures. She takes her appearance from the dragonfish, a fierce predator despite its tiny size, a mere 32 cm. Naturally, reflecting this, Lantern's species of merfolk tend to remain on the small side, being 5 feet in length at the very most. However, in comparison, Lantern is a giant, clocking in at around 15 feet long from head to the tip of her tail.
Lantern typically would be far out of sight, in the parts of the ocean where light can't penetrate, where she is almost invisible. However, angry seas and pure coincidence did she wind up washed ashore. Fortunately, unlike other deep sea fish, she lacks a swim bladder that would exploder at lesser pressures. And, of course, she's amphibious, sporting a pair of lungs in addition to her somewhat feathery, visible gills.

Starting at the top of her head, Lantern has unusual grey-black hair that comes to about her shoulders. Her eyes are silver and bear lashes of the same color, with patches of bioluminescene underneath them that act as headlights so she can see in the murk of her natural environment. The rest of her face seems fairly normal, until you take a closer look at her mouth. Her lips are black and pouty and perfectly kissable, but it's noticed that her mouth doesn't quite end where it should, faint lines stretching almost to the back of her skull. Her mouth is capable of opening to a normal width to allow to speech and consumption of normal food, but can also open wide on spring-like muscles, jutting out and forward to snare prey in less than a second. With so little food in the deep zone, there's no room for error. Her teeth are long and hinged, springing forward when her mouth is open, but folding back smoothly when closed.
Her lower jaw also has a strange, thready protrusion stretching about 8 feet in length, the end glowing a pleasant bluish color. This is used to lure prey towards her mouth, because she's lazy and can't hunt well to begin with. Her neck is long and slender, with redpink gills right behind her jawline, and the rest of her human torso is what you'd expect of a mermaid, with long arms with delicate fingers. Her skin is a greyish blue color, ending at her 'hips' and blending into a positively naga-esque tail. It is long and scaly, sporting a short dorsal fin from the top down, and then a smaller fin before ending in a typical fish tail. Bioluminescent spots line her body from just under her armpits to the tip of her tail.
She moves around on land with serpentine movements, quick in short bursts, but otherwise sluggish.

Despite being a predator, Lantern is a kind soul and appreciates company of any sort, especially so far from home. She loves having her (somewhat slimy) scaly body rubbed and she'll love you forever if you offer her your backyard pool to splash around in.
If you're rude to this literal fish out of water, you'll find yourself giving internal belly rubs in the blink of an eye.