Ok, something to keep in mind, "ghosts" don't exist in Shadowrun

What was once thought to be "ghosts", have been proven to be actual spirits, its just that before the awakening, many spirits couldn't materialize on the physical plane, and could at best (if they were very powerful) Manifest...
A Manifested spirit has no physical body, but can speak, move through objects, and other "ghostly" interactions.
Since the Awakening, these "ghosts" have been revealed to be actual Spirits (Free Spirits, Shadow Spirits, 'Dark Spirits')....
With that in mind, it would be a Free of Shadow Spirit that is "Haunting" your player - which is entirely possible as Spirit motivations are a varied as the motivations of People

But this also means your mage character could have ways of dealing with the Spirit that you don't like.. Simple astral combat for example, or even just Banishing... So keep that in mind.
Street Grimoire has info on Shadow and Free spirits, but more info can be found in previous mage books like 4e's Street Magic.