imu (one piece)
Heavy spoilers for episode 885/chapter 906 and later follow here:
Imu is the secret occupant of the "'Empty' Throne", making them the highest-ranking member of the World Government above even the Five Elders (a.k.a. Gorosei). As a result, they are the overarching antagonist of the whole One Piece series.
Very little else known about them (their sex, species, age, full extent of their powers and abilities, origins, and so on), as their identity were kept secret until the very late stage of the story and they were yet to be fully revealed. However, in their first reveal, they appeared as a silhouetted hooded figure with distinctive ringed red eyes and a very tall crown.