General:
* Regent: A person who rules a country during the absence or incapacity of the monarch.
* Viceroy: A representative of a monarch who governs a colony or province.
* Protector: A person appointed to protect a country during a period of instability.
* Governor: A person appointed to rule a region or territory.
* Administrator: Someone temporarily in charge of an organization or territory.
* Interim leader: A temporary leader who holds office until a permanent replacement is found.
More specific:
* Lord Protector: A title used in England for a temporary ruler who is not a monarch.
* Curator: A person appointed to manage and care for a museum or other institution.
* Trustee: A person appointed to manage property or funds on behalf of others.
* Custodian: A person who is responsible for the care and safety of something.
The best word to use will depend on the specific situation and the nature of the temporary rule.