As the title says, im looking for protagonists who has a strong potential for something but hides their strength due to a particular reason. Its been so long since ive read/watched something like this XD
Action/ adventure/ fantasy genres preferred but it doesnt have to those genres.

Right now '17-Kiss Sai to Dilemma' comes to my mind. It's a simple shoujo, so maybe try it out.

I'm not sure if I wanna do this

I know Mahouka Koukou no Rettousei (Novel) has one. I know there are some manga series related to the novel and an anime came out a while back.

Black Haze,maybe?

Iris Zero is a great one. Shame about the hiatus though.

Tsubasa Reservoir Chronicles - Fantasy shounen, hidden until halfway into the story
Kuroko no Basuke - Basketball shounen, revealed fairly early
Noblesse - Vampire webtoon, not 'hidden' exactly, but their powers are never fully exhausted
Yanki-kun to Megane-chan - School-life and gangster shounen, hilarious reversals
Superior - Fantasy shoujo

Seconding Noblesse also adding Densetsu no Yuusha no Densetsu and Hyouketsu Kyoukai No Eden