I find that having some other entity that, while part of your character conceptually, is separate from their person, adds a lot to an RPG... Whether it's an Engineer's drone, a Warlock's summon or familiar, or just a squirrel that's tagging along, it's clearly an entity that isn't the character itself. In Fate, however, I'm not so sure how to handle that. I've had several models in mind, but I need some guidance here:
1) Pure flavour. It's only in the agreed-upon narrative, and nowhere else. It doesn't interact with combat.
2) An aspect. It can be invoked and compelled, and obviously an important part of your character. It's also always true, naturally.
3) A stunt. It acts as an extension of a character's abilities, or the equivalent of a massive stunt a PC can pull off once per X.
4) A set of approaches or skills, on a separate sheet. Sure, the character can be Flashy or Sneaky, or they can Craft and Notice... but their pet can Corrupt(+3) or Chase(+2). It acts instead of the PC, on their turn.
5) A full character with stress, skills/approaches, and consequences. The question here is this: should it act instead of the PC's turn, or in addition to it. It would be unpleasant for there to be up to twice as many acting entities as PCs, right?
6) A non-combat entity with maybe a couple of aspects (or not) and no skills, that can't act in combat, but can be commanded by the PC to scout ahead, send messages (carrier pigeons?) and perform minor feats using the PC's skillset.
It doesn't have to be one of these, naturally. I just provided some half-answer of my own, I'm looking for a model more than I'm looking for a poll on my own ideas.