An emperor's main aide

Imperial, Royal or Galatic:

Regent,
Proctor,
Elector,
Freiherr,
Dominus,
Vidame,
Seigneur,
or Jonkheer, to name a few.

Ot perhaps "Lord Commander of the Home Ship".....no.....I have to stop.

The captain of the vessel is still the captain. The emperor may be an esteemed passenger whose presence on board may designate this ship as the flagship of the fleet. A visiting dignitary (princess) is still another (albeit temporary) passenger. The likely protocol might be to have the Executive Officer (Captain's immediate subordinate) meet the dignitary upon boarding, and introduce her to the captain. The captain would then escort her to an audience with the emperor. Remember, the captain commands the ship (not a passenger, regardless of rank). A captain may receive orders from his chain of command, but until relieved, the captain commands the ship.

Oops, sorry . . . I guess I imagined her arriving via some form of imperial space transport. If she's already planet-side, you could use some sort of designated pomp-and-protocol minister to see to all the ceremonial niceties, escorting duties, etc.

Prime Minister
Chief Vizier
Crown Prince/ss
Chamberlain

Or any highly ranked person at court in the Emperor's favor (nor not is his favor if the Emperor doesn't want this to succeed).

I was about to suggest "Chamberlain", and I see ULTRAGOTHA has beat me to it. So two votes for that one.

You could also make up a title of your own.

caw

Sat Nam! (Literally "Truth Name"--a Sikh greeting)

Another vote for Chamberlain. But any of the people on Ultragotha's list would work.

Blessings,

Siri Kirpal

There are two different ways you might approach this.

The first way would be to assign a courtier to her. This wouldn't be a paid position-- it would be more like assigning a highly-ranked person at court to be a social friend. This friend, presumably, would be someone who would always keep the emperor's interests at heart... and would probably pass on information gathered to the emperor's intelligence.

The second way would be to assign a functionary to her. This is the more bureaucratic approach. The person may or may not have rank, but it would be their job to receive guests and make sure all was correct and that schedules and protocol were followed. It might be someone like a majordomo, or perhaps a steward.

I'd go with Chamberlain too - the person who is the head honcho for the day to day running of the court.

A Chamberlain is a good possibility, or a Grand Master of the Household (who would typically be one of the high aristocracy). Or -- does the emperor have any siblings? Sending a royal brother or sister to greet & escort the visiting princess would be a very great honor to her.

Royal siblings is a good suggestion, heir to the title perhaps if they're adult?
High Chancellor?

It's your tale, so you get to tinker with the etiquette rules, but a reasonable rule of thumb would be to send someone of similar social/aristocratic rank, unless you want to deliberately offend your princess, in which case you send someone more junior.
I would add another vote for Chancellor - on the presumption that such a senior role in an Imperial setting would probably be a senior noble (or royal sibling as benbenberi suggests ) that the Emperor trusts with everything.

Courtier would be the general term, more commonly a descriptive term than a title.
Chancellor usually implies someone with very senior management/administrative duties, such as a head of government of the equivalent of a justice minister or foreign minister.
Chamberlain has usually been the title of someone running an imperial/royal/noble household, rather than the government, so it's probably not the sort of position you're looking for here unless it's more of a personal rather than official visit.

In your case, given the rank of the visitor, the appropriate escort would probably be the equivalent of the foreign minister or even prime minister, so chancellor would work well for the title, as would vizier (for a minister) or grand vizier (for prime minister).

Well its a dark idea because of the implications (genital mutilation and all), but does your space empire have eunuchs? If so, then maybe a chief eunuch would be your guy. In places where they were used, eunuchs were the ones that handled affairs that involved royal ladies. They were considered more trustworthy around women (for obvious reasons).

Really, it depends on how you want your empire to be in terms of titles. For example: want to make it "minimalist"? Maybe just limit yourself to Chancellor, Regent or Vizier. Want to go on grandeur and pomp? Maybe try for a bit of a tirade like Viceroy of XXXX or Lord Protector of the Imperial Office of State.