Here's how I described it on another thread, with a little bit of extra info
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Essentially it's a part human, part shark type creature. Not like a mermaid where there's tailed bottom and then a human upper body. The creature has legs, a long tail that can change it's shape, a wide triangular shaped heads with black eyes, long fingers that end in sharp claws. It's back has this function where it can inflate or deflate with air depending on if the creature wants to sink in the water or float near the top. I picture the thing on it's back as a replacement of the dorsal fin, instead of just one fin, it's like a triangle shape going down his entire back until it gets to his tail, if that makes sense?
It's stomach is white and it's back is a dark blue with ripples of lighter blue running through the color. This main shark creature has their version of 'freckles' except instead of darkened spots, this creatures has many white spots on his back from hanging out near the surface too often. At one point, my human MC describes them as constellations (white spots on the dark blue skin, it seemed fitting if cliche.)