WHAT IS THIS FISH

backstory: I moved into this house to a neglected tank and decided to take over it's care.
I have never managed an aquarium before, have enjoyed the process and probably watched 30+ hours of content to educate myself.
couple weeks ago i ripped everything out, removed substrate, redid with sand and some living plants and driftwood. cycling is all done and it's stable. i wish i had put substrate under the sand but I didn't know. did add root tabs. i'm looking for a new tank that isn't scratched where I can put a substrate and move everything over. anyway.

now I'm trying to stock the tank properly.
currently there is 2 x-ray tetra, 3 buenos aires tetra, 3 bronze corey, and this fucking guy.
i'm planning to buy 1 at a time each type to increase their schools so they're happier and not to disrupt whatever heirarchy they've already developed being together for many years. frankly the coreys seem happy, i might not get more of those. focusing on the others.
i've already added neo shrimp and ramshorn snails.