Ethereal Light (Beyond Expectation) | mha bkdk

"
But this, mind you, was no thing. And the merman was quick to realize this when it moved. Bright, sun kissed fins struck the water, pushing sloppily to urge the rest of the attached body to sink. The- was that a human? Was Katsuki witnessing the first human up close and personal? No, surely there had to be others. But if there were, then stories would have surely been passed, which meant that they were slain.

The human didn't have long, protruding claws like Katsuki had once thought, nor a pelt made of spears and iron. It lacked the pearls and jewels the royals were rumored to have- No, instead it was a darkened blue, almost black, with toxic green strips lined down the sides. Humans weren't supposed to have fins, being land dwellers, but this one did. Were they evolving? Katsuki was struck with anxiety at the thought of this. Were more to follow?
"

or;

Katsuki is a merman and Izuku is a human, both are scared of the other.