Historically, this person was known as a steward. They were exclusively men, but I don't think that matters given the nature of the story and there is the feminine "stewardess."
The term is suitable for the fictional world you describe.
OED
steward
1.a. An official who controls the domestic affairs of a household, supervising the service of his master's table, directing the
domestics, and regulating household expenditure; a major-domo.
Obsolete exc. Historical.
a1616 If not, let me see thee a steward still, the fellow of seruants.
W. Shakespeare, Twelfth Night (1623) ii. v. 151
Also
5.a. One who manages the affairs of an estate on behalf of his or her employer. [(sometimes in the absence of that employer) my
addition]
1892 The estate had been so long without a head, under the management
of the steward. F. P. Verney et al., Mem. Verney Family Civil War
vol. I. 14
The Scottish term is Factor.