Good question! Twintails has no opinion about whether her appearance makes sense, but the humans are free to form one! Here's what they've had a chance to notice so far:
She is very small. By height and width, she looks like a 6-year-old child; but her proportions suggest that she's more mature than that for her own species.
Her upper body looks basically human; the skin looks like normal human skin. She's Ambiguously Brown; without any other examples to go on, it's unclear if this is an indication of some kind of mermaid ethnicity, or if this is just the universal mermaid skintone.
She has dark brown ?hair?, which is somewhat wavy and hangs down to the ends of her ears. It looks normal from a certain distance, but by now the humans are standing close enough to see that it's not actually hair. The strands are too thick, for one thing. And they… seem to move on their own, a little bit, especially when she's agitated. (A human from her own world might draw comparisons to Medusa, though Twintails's 'hair' isn't nearly that thick.)
Her lower body is scaly and silvery. If the humans' concept of a mermaid typically has just one tail, it will probably stand out that Twintails has two. Sort of. About halfway down, or maybe a little more, her tail forks into two ends. It… doesn't really look like it's supposed to be like that, though; one of the tails looks a bit… supernumerary.
She's missing an eye. (Either that, or she's been keeping her right eye persistently closed for no apparent reason.)
They're standing close enough to see that her arms have some thin, faint scarring, like something was digging into the skin in a lot of different places.
It's actually not clear whether she's a girl or a boy. She's completely naked, and by now they've probably seen her from nearly every possible angle — with all that flopping around she did earlier — but she has no immediately obvious genitalia. Meanwhile, her torso looks like a buff man's. (Although notably there are no nipples.) If they're thinking of her as a 'she', it's probably mostly because of her voice and her hair length.