In the One Piece series, characters with advanced knowledge and training in Haki can sense and perceive Haki in others. However, Haki itself is not something that is physically seen with the eyes but rather sensed with one’s “spiritual” or “willpower” perception.
There are three main types of Haki in the One Piece world:
- Kenbunshoku Haki (Observation Haki): This type allows the user to sense the presence, emotions, and intentions of others, even if they are hidden or at a distance. Skilled users of Observation Haki can predict their opponent’s moves in battle.
- Busoshoku Haki (Armament Haki): This type of Haki allows the user to create a forcefield-like armor around themselves and their attacks. It can also be used to bypass the defensive abilities of Devil Fruit users. When strong Busoshoku Haki is used in battle, it can be visible as black or dark-colored streaks or a coating on the user’s body or weapons.
- Haoshoku Haki (Conqueror’s Haki): This is the rarest form of Haki and is a manifestation of one’s willpower to dominate or intimidate others. It can be used to subdue or even cause individuals with weak wills to lose consciousness. Haoshoku Haki cannot be seen but is felt as a powerful presence.
Characters in the One Piece world who are proficient in Haki can sense the use of Haki by others and can react accordingly, even if they cannot physically see it. It’s an essential element in battles between strong characters and plays a significant role in the series.
According to Oda, Haki is invisible but for viewers Oda did colour black so people can see it.