He is the 33rd son of Big Mom and a user of Armament Haki ... and that is Observation Haki. He utilized this skill actively in the Wano Country arc, although he had no real control over it.

As the Straw Hat captain, he’s had to level up his skills to take on tougher foes, and learning Observation Haki, Armament Haki ... making him a real threat to the Yonko. Conqueror’s Haki ...