Regent

Regent is a title given to one who rules on behalf of another, typically because the actual ruler has not reached maturity, or is absent or infirm.

In 2267, a power struggle fomented by a Klingon agent left Leonard James Akaar as Teer of the Ten Tribes from the time he was just hours old. His mother Eleen ruled as his regent, and signed a mining accord with the Federation. (TOS: "Friday's Child")

Palamar was lead by a regent during the 2370s. (DS9: "Business as Usual") The Evora were led by Regent Cuzar in 2375. (Star Trek: Insurrection)

In the Mirror Universe, the regent was the ruler of the Klingon-Cardassian Alliance. Worf was regent in the 2370s. (DS9: "Shattered Mirror", "The Emperor's New Cloak")

Oxilon was the Council Regent of the Talaxians encountered by the USS Voyager in 2377. (VOY: "Homestead")

In the 2400s Rodrigo Doenitz was the Regent of the Union of Imperial Planets. In 2408 he ordered the ISS Rosenante under the command of Captain Ahmed Ibn Salid to destroy the planet Sunali II aas well as a massive task force of the KCA which was assembling in the system. To facilitate this Salid was given the Omega Device.