What would you call a person who holds the position of emperor but doesn't have the official title? For example, in my story the true inheritor of the title is too young to rule the empire, so one of his uncle's steps in and takes on the position. I want to give the uncle at least some title which distinguishes him from his other brothers, otherwise the brothers have kinda the same status and power as him. Same with his wife, she should also have a new title. And yeh, this is fantasy stuff so the titles doesn't have to be rooted in history.
In general, this is called a regent. However, some systems have more specific titles. Like Interrex (used by the Polish-Lithuanian Commonwealth and the Kingdom of Rome). Depending on how you want to build this political system, you could also choose to pin regent-duties onto an existing position at court that the uncle had already been appointed to. Maybe Chancellor, or Constable of [insert nation name here].
I like Chancellor. It reminds me of Star Wars, how Chancellor Palpatine became Emperor. If your "temp emp" character is planning on seizing power and becoming "perm emp" then that could be a very nice nod that serves to foreshadow a little bit.
