Hmm... well a spirit of the city could be one of two things.
It could be an individual personality, a demi-god that has existed ever since the first sparks of civilisation in the area. It would be a normal sponsor in effect, one that is tied to the city (probably feeds off of emotion, or sacrifices in it's name (muggings etc)) and it would have it's own wants and needs, mostly tied in to it's survival and entertainment. It could be bargained with and it could lend it's power just like any sponsor.
This spirit would have an emissary that would help it feed, or perhaps if the city is failing then it might have an emissary to find some way to to reinvigorate the city - maybe related to a key industry or area of the city. Or maybe some key buildings are to be demolished or... you get the idea. Alternatively he could be hunting for a serial killer (if the city spirit feeds on positive emotion) or he could BE the serial killer (if it feeds on blood).
Alternatively, the spirit of the city could be a primal force, an animal mind created over hundreds of years of human hopes and fears, a conglomeration of every emotional event: Serial killers, new born babies, car crashes, rock concerts, homeless shelters - all of it. This wouldn't be a sponsor you could bargain with, instead it'd be a savage elemental and tapping in to that power would be overwhelming.
This kind of emissary could be anything: some kind of traumatic event could have allowed him to tap in to the city spirit and now he can do it all the time (lifting a car up by himself to save someone trapped underneath, racing through a burning orphanage to save the kids etc), he could have been homeless for years or he could be a kid, born exactly 100 years after the spirit first coalesced.
Either way, it'd be fun to make up powers