Kim Cheol? If you mean Sung Jin-Woo, the protagonist of 'Solo Leveling', his power set is arguably the core of the entire series' appeal. It's not just a single ability but an evolving system.
Initially, he's the weakest E-rank hunter, barely stronger than a normal human. The pivotal moment is his gaining the 'Player' system after surviving the double dungeon, which lets him perceive quests, stats, and a leveling interface. This is his foundational power: a gamer's growth mechanics applied to reality. He can see his HP and MP, accept quests for rewards, and most importantly, grind.
The signature ability unlocked from this is 'Shadow Extraction'. After defeating powerful foes, he can raise them as undead shadows, retaining their skills and strengthening his personal army. This shifts him from a lone warrior to a commander. He also gets an 'Instant Kill' skill against much weaker enemies, 'Domain of the Monarch' for area control, and later, the ability to store and use countless weapons from his inventory. His evolution from a fragile human to a literal monarch of shadows, capable of rivaling the absolute powers in his world, is the entire narrative engine.
What makes it unique is how it synthesizes RPG mechanics, necromancy, and a classic underdog-to-overpowered journey into one cohesive package.