Bob can easily be:
- An aspect
- A stunt
- an NPC
- a PC.
- A tool which grants a skill
Which is best varies by what role he's to play.
Bob as Character
Bob, as a character, has an unpleasant set of aspects to cope with:
- Lives inside a token (the skull)
- obligated to serve the owner of the skull
- allergic to sunlight
- porn addict
- Brilliant but unprincipled
- bodiless spirit
- can't work magic
He likewise has only a few skills, but we can say clearly he's Lore +5, and probably Scholarshi +4.
The behaviors when owned by Butters are pretty much the same as for Harry. We really don't know enough about his time with Kemmler to judge, but the Kemmler-Chunk Evil Bob certainly shares most of them.
As an NPC, Bob's far less interesting, and really would be a waste.
Bob as seen in the novels is a playable but very limited character. As seen in the TV show, he's much more interesting, tho' limited in very different ways.
Bob as Tool
Bob as a tool is a Lore skill that can only be used in the presence of the skull, and only for knowledge checks.
this overlooks Bob's ability to ride the cat.
Bob as stunt
Bob as a stunt is likely to be equivalent to a library in and of himself. So, one can do research using Bob alone.
Harry does so in several books. Including Cold Days. But again, this ignores his abilities to ride the cat.
Or he could be a +1 to magical actions across all three magical skills. Bob does certainly prop up Harry's Discipline on a few occasions.
Bob as aspect
This is a weak yet very broad application. Bob can be used to possess animals. He can be used in Lore checks, or Thaumaturgy. Or even Alertness.
Harry uses knowledge from Bob for purposes including all the following skills: Alertness, Burglary, Contacts, Conviction, Deceit, Discipline, Intimidation, Investigation, Lore, Presence, Rapport, Scholarship, Suvival.
But, as an aspect, Bob can be used to compel the player. Or worse, to declare that "Bob was wrong!" and take away from the role. Which isn't unprecedented in the novels, either.
Which to use?
It depends on what you want to do with Bob in the game.
- If he's just a color bit to get bonuses, he's a stunt.
- If he's unreliable color, he's an aspect
- If he's a participant in the plot, he may be best as a character, even a PC.
- If he's just a penalty avoidance, he's a tool or stunt.
Or, you can have him be two or more as needed. From the way Harry is affected, he's a character and an aspect and a tool. Some times, he's important (his player is there), others he's just a wizard's audiobook (his player is absent).