Matt is a merman. His people come from the deep pitch black depths of the sea where being blind is the norm and they navigate by sense of smell and changes in pressure.

I can see this going a few different directions. Little Mermaid!AU where Matt ventures to the surface and falls in love with Foggy. Mer!Stick does not take kindly to Matt wanting to escape the underwater war he's been training for.

Merfolk are known. Deep sea merfolk rarely come to the surface let alone live on it. Matt is something of a curiosity to surface-dwellers which he hates. Until one day, he meets Foggy who doesn't treat him like a freak.

Or take it in another direction, anon. Just gimme some deep sea mer!Matt :)