Using GURPS Magic.
Requires Bind Spirit (M158); a 225cp ally can be summoned as a service but to if you want "Plot Immunity" you may want to pay it with points. No plot immunity, then the GM hides the character sheet and rolls in so many forsaken tables for its motives and hidden desires... which is actually more interesting.
Ghostly Companion Template
-14 point template
Advantages : Doesn’t Breathe (B49) ; Doesn’t Eat or Drink (B50) ; Doesn’t Sleep (B50) ; Insubstantial (B62) (Usually On, -40%; no-need for flight, can move any direction at full move) ; Invisible (B63) (Substantial Only -10%; Visible to Mirrors -10%) ; Magery 0 ; Night Vision ; Resistance (B80) (Immunity to Metabolic Hazards) ; Unaging ; Unusual Background ;
Disadvantages [-205]: Social Stigma (Valuable Property) [-15]; Dead Broke [-25]; Supernatural Features (No Body Heat, No Shadow, No Reflection) [-25] Dependency (Common, Ambient; Constant x5 cost) [-50]; Dependency (Ties to the Mortal Plane, its destruction causes constant loss ) [-50]; Unhealing (Partial) [-20]; Vulnerability (Magic Weapons, x4) [-20].
Skills: Typically has the skills of a Mage Template
Class: Undead, Spirit,
Non legal human Entity meta Trait [-40] - Social Stigma (Valuable Property) [-15*]; Dead Broke [-25];
*Social Stigma: Monster [-20] are for creatures that are not owned, which means anyone can destroy without social repercussions. It may be even encouraged like pests.
Comfortable ($5600/month)ST: 9 [-10] HP: 9 SPD: 4.5
DX: 10 Will: 14  Move: 4
IQ: 10 Per: 10
HT: 9 [-10] FP: 9 SM:-0
Dodge: 8 Parry: - DR: -
Typical Age late 30s and early 40 (13-20 years of work)
Advantages : (No need for Unusual Background); Magery 15 (Ally Accessibility -40%; Half-Power unless casting at Night -20%; Pact: Sense of Duty to Coven -10%; Pact Code of Honor Soldier of the Faith -10%) ; Signature Gear: Powerstone 15pts ; Powerful Familiar ;
Disadvantages [-20]: ST-1 [-10]; HT-1 [-10]; (you can add Enemies, Dependents, conflicting Duties, Inconvenient Compulsions, etc...)
Skills : Thaumatology-31  (-6 for other colleges - prereq count); Enchantment-25 
The difference between the Deathless Companion and the Demon, the deathless companion is more of a slave while the Demon is more of a Partner. Less powerful than the Demon, they create less complications for the Mage.