yes, i like this kind of theme. At some point in each of this manga/manhwa these guys unlock some of their powers. I include gantz and hajime no ippo since the beginning these MC is not really the type to fight, and in some aspect they unlock their strength
Legend of Maian The Ruler of the Land Kingdom of Zombie The breaker / The breaker: new waves King of Hell TAL Special Martial Arts Extreme Hell Private High School Crepuscule (Yamchi) Gantz Hajime no Ippo Akame ga Kiru Seikoku no Ryuu Kishi Tower of God Break Blade Shishunki na Adam Witchcraft Works Last Ranker - Be the Last One Madan no Ou to Vanadis Hagure Yuusha no Kichiku Bigaku Hidan no Aria Rosario to Vampire / Season II Strike The Blood Nanatsu no Taizai Seiketsu no Haguruma
I think king of hell really fits, Because he is stuck in teen form which is quite weakened, and his unlocked form is his full adult form which looks cool and powerful too.