Figurehead

In politics, a figurehead is a grownup who de jure in hold or by law appears to advance to an important as well as often supremely effective title or office, yet de facto in reality exercises little to no actual power. This ordinarily means that they are head of state, but non head of government. a metaphor derives from the carved figurehead at the prow of a sailing ship.

As a derogatory term

The word can also produce more sinister overtones, in addition to refer to a powerful leader, who should be exercising full authority, who is actually being controlled by a more powerful figure behind the throne.