Are there rules for Creation in any of the books? If not, has anyone ever done it, and, if so, how did you do it?
No. And with great care and forethought.
You're going outside the box here, which is fine! But as a SG I'd be hesitant to give a Player control of a ghost as a character. To me, ghosts have "obscure" motivations and priorities, and not every player could do justice to such a potentially powerful and unbalancing character. (Of course, if it's the girlfriend/boyfriend of the SG who's asking, then the SG is just screwed, natch.)
~IF~ someone were to put a wand of POF to my head and force me, I'd do the following...
Look at the various examples of ghosts and spirits in the various published sources - there's one in the core book, and others scattered around. Then I'd compare those powers and weaknesses to the +/-10 Virtues/Flaws of a Companion, and try to balance them out appropriately. I'd also put some "undefined" limitations on the character - perhaps what they can effect, or who can see them, and when, and so forth.
If they have any powers, they have to consider their Magical Might - run out, and it's game over for the day.
And (me), I'd make it clear that any "character improvement" would be at the whim of the Story Guide and/or the Troupe, emphasizing that Ghosts generally don't learn, but are stuck at the moment of their death.
And I'd want a "goal" for the ghost - why it's still here, and what is necessary to lay it to rest, for better or worse. If it's a "guardian spirit", then I'd never let that be for another PC - NPC only. Two PC's would team up to be unbeatable - unless that's your idea of a good time, in which case knock yerself out.
Me, I'd discourage it heavily unless I knew from experience that I could trust the Player to add to the story, and not abuse the possibilities - it ~is~ a power gamer ploy. If the player was set on it, I'd question "why?", and get a feel for where they wanted to go with this, and hold them to that!
And good luck!
I don't know of any place with rules for this. I'd take a shot at it by modifying the infernal beast rules in RoP:I (providing that it isn't the ghost of a Magus).
Which raises an interesting question. Magi and ghosts...
Now I imagine it is extremely rare for a magus to become a ghost, presuming those that pass into twilight and roam the cave of twisting shadows etc are not exactly ghosts.
The other thread about essential nature grabs my interest in this one too.
A ghost presumably IS a ghost, a distortion of a once living person but with distictly new powers motivations and abilities. I imagine that apart from the residual motivational factors from its former life there is no great necessity for it to retain any of its other. This would mean that the ghost of a magus couldnt necesarily have to retain its hermetic powers; even if it did retain some of them they might be highly distorted but perhaps it 'could' teach some things from memory.
All this, of course, works on the assumption I haven't missed something central about magi and ghosts somewhere in the rules.
The adventure/mini-saga book "Broken Covenant of Calebais (Revised Edition)" has rules on ghosts, ghosts-of-magi (, and elementals).
A summary of those rules, and rules for becoming a Living Ghost , is in The Mysteries Revised Edition.
You know, now you come to mention it I do recall reading something in the mysteries revised about that. There was a lot of information in it, much of which I would never get round to using so I skimmed it.
But that only applies to certain types groups though.
Never created a Ghost Companion, but I did create a Companion with a Faerie Friend. In my mind, it is better to create a traditional Companion, then create a sidekick (given appropriate virutes).
The Companion herself was a Seamstress/Merchant. Simply wasn't a lot of story potential there unless we roleplayed the haggling, or getting hit on in the town bar (ok, so there WAS potential, but compared to the other companions who all had something a "little special", she was rather mundane).
So the Troupe discussed it, and she ended up with an ugly little goblin with +10 Might. It could turn invisible (1 might point, I wanted it a common trait since she was always dealing with mundanes), and teleport anyplace within sight (4 might points, so there and back again once a day). I also allowed the Troupe to assign only half of the skill points, leaving me enough leeway to add depth and surprises later.
It was quite fond of the seamstress, did its best to please her, but had its own mischevious and uncontrolled personality, was never played by the same player that played the seamtress, and was a great story hook.
Even if your player is a power gamer and just wants to be a ghost because "it's cool", I think this is a better route to go. Then you sidestep the whole "ghosts never change or improve" rule, which will quickly discourage both power gamer and earnest roleplayer alike.
Why so?
Though I've been known to create Infernal ones, Ghosts, by the rules, are typically of the Magical Realm.
Being magical in nature, there's a strong arguement for ruling that Magi (or anyone) who die in Arcane conflagrations or disasters could become accidental ghosts.
Magi/People could have their body disintegrate, but all other parts of them remain to wander the earth (or haunt the site of the event).
They could have their body turn to dust, the soul go to the afterlife, but have their intellect and emotions remain as a reflection of their existence.
They could interact with visitors, unable to comprehend what happened, or completely ignore visitors, instead acting out the same events night after night (or month after month under the full moon, etc).
Magi as ghosts is a compelling story.
And yes, there's rules for it in The Mysteries, Revised Edition.