Regent

From Memory Alpha, the free Star Trek reference.

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")