And what an interesting idea to start with. I like that you already know that you don't want a ghost or a Daimon.
I think the best fit looks to be one of the Spirts of Sensation, Realms of Power, Magic, page 105. If your character is musical, perhaps the spirit of a given song, or the sound of a lyre. It may even be the spirit of an incantation, perhaps the words used to draw upon your favourite spell. Or the sound of distant thunder (perhaps an enemy to the sound of thunder overhead...).
Two habbits you need to break in order to have a successful Ars Magica experience
In My Humble Opinion that is.
First, you need to break away from the concept of a GM. He is now an SG, or rather an ASG (storyguide/alpha-storyguide). This position is not the gaming god of other systems. Rather, he is just one of many players, a first amongst equals. He should have his own character in the game (a PC, not an NPC), and other players should take turns running adventures (beta storyguides).
Second, books should not be off limits to other players. How are you supposed to run your own adventures? You should all be able to separate player knowledge from character knowledge. Yeah, you read RoP-Magic cover to cover, but unless your character has a score in Magic Realm Lore, you can't use any of it in game. And, from my own personal experience, having multiple players with access to the same books smooths over any rules questions. Multiple minds can bring a greater understanding.
As for a cool idea for a spirit familiar relating to MuIm, I suggest a shapeshifting Jinn with powers of illusion and deception
Or a psycopomp, a spirit that escourts the dead to the afterlife.
I posted a concept I kicked around for a raven that visits artists before they die. He's a muse and grants the minor virtue of Free Expression on top of it. But the artist always dies before completing the work.
A djin though is right up your alley though, MutoIm and Aribic influence.
This bit is very much up to the individual gaming group in all of the tabletop games or Ars I have played or run there has been 1 GM for the entire campaign with no changing over and that person running the universe.
I tend to agree with the bits about books although If I was in a postiion to do so for most campaings I would probably prefer players not to read some books (Ancient Magic, Hedge Magic) as I may want to use some elements of those books as a suprise