Regent

From Srafopedia

A regent, from the Latin regens "who reigns", is a person selected to act as Head of state (ruling or not) because the ruler is a minor, not present or debilitated. Thus, the common use is for an acting deputy governor.

(Source: Wikipedia)

In The Amber Spyglass, Metatron is revealed to be the regent of The Authority, who is too old and frail to govern.