I'm not sure it necessarily needs to be possession, but lots of beings we might think of as spirits have changelings/scions, not spirits of intellect.
Half-Fae/half-humans are changelings, mostly human until they Choose. Glau in PG is a djinn/human scion, and he's still mostly human, not a spirit.
So I think a supernatural being in nonphysical/pure-spirit form, or a bodiless shadow of a greater supernatural being, would have to be one of the parents.
Fae seem pretty tied into the Changeling/Choice mechanic, so I'm skeptical of Mab or Lea. Also, if Bob were Mab's son, I'd expect him to be incorporated into Winter's power structure, not essentially exiled and fearing for his life.
What about Odin (possibly with Gard, who might be mortal enough, yet is over 1000 years old)? He has ties to Winter as Kringle, but isn't of its power structure, Kringle is still Wyldfae. Odin is a god of knowledge, writing, and magic (among many other things).
Odin seems physical, but we really don't know what he is *under* his Mantles. Even if he is basically physical, he might be able to do something like the 'shadow' effect. IMO Valkyries are sort of like the Faerie Knights, so Odin probably can split off part of his power.