Hello, I'm looking for stories in whatever medium (manga, novel, anime,whatever) in which there's a protagonist (or deuteragonist) that acts like they're useless and can only eat, sleep and play, but in reality they're reeeally skilled.

Bonus points if there's a character by their side who's admired by everyone because they're strong, but said character knows the mc's stronger than them

I'm thinking more of martial arts stuff, but whatever you got is good

Iris Zero I think has that feel to it. Not martial arts though. 😛
Gosu except it doesn't have the bonus. 😛

kamisama dolls might fit. Takes some time though
the new gate rough fit, fits the bonus
hataraku maou-sama! Feel like this one is a stretch and its more comedy than anything

No Game No Life. Novel, anime & manga. Although the argument can be made that the MC really is useless.

Phi Brain - Saigo no Puzzle . Anime & manga. I only saw part of it. Not sure about this one.

Kill No More. Novel. Sidekick fits.

Your description sounds really familiar. I'm going to have dig for titles.
Edit. Found it.

Daraku no Ou (Novel). The MC is the Demon Lord of Sloth, so he is both useless and a badass.