Personas are composite constructions that serve two distinct purposes: to act as aids for design decisions, and to generate empathy. The two can easily get in each other's way: people looking for information that they can act on might consider all the additional texture of people's lives "fluff", whereas people for whom a persona doesn't represent a specific set of information but, rather, a set of contexts and behaviours might be disappointed if the persona had "just the facts, ma'am".

Within corporations, personas are an intensely political artifact: claims on what they should contain are made by many different groups of people: marketing, product planning/management, designers, researchers, engineers.. Each has their own set of questions they want a persona to answer, and each has their own claims on what the persona means. Our poor hardworking persona, then, has to act both as an embodiment of consensus and as a coordinating artifact - something that various kinds of people can look to for guidance as they go about their work.

Regardless, personas are usually composites: rarely in research are personas constructed from one perfect respondent - they are amalgams of many stories and many situations. This is (hopefully) not the case for personas that are entirely behavioural, and who are being constructed for a highly specific activity or around a particular product; the necessity for composition arises when generating personas for a wide variety of activities or products, and especially the case when personas are created as part of generative research, which tend to include attitudinal and aspirational data (to use marketing terminology) in addition to behavioural data. The point is to create a person who embodies the entire range of behaviours and attitudes for that class of people - so that the resulting design covers as much range as possible.

It is at this point that something like the following occurs:

"Does he have a car?"
"Uh, I don't know. I mean, he has an iPod.. isn't this the persona who doesn't like CDs because he wants access to his music all the time?"
"So, ok. He has a car, and he listens to his iPod in it and his FM connector keeps getting interference but his car stereo is old so he has no other way of connecting it"
"Yeah, that sounds about right"

If you're a ux/design researcher you've probably had a conversation very much like this. This is the act of composition: when the essential stories & features have been picked, and they have to be woven into a single story. In this sense, personas are a collection of fictional facts: the stories come from different places, but they have to be made part of this person's life, reflecting their desires, their interests, the conditions of their life and so on - and in the process are given a fictional form. At all times the data speak, but through the voice of this character. In this sense, defining personas seems remarkably similar to writing a novel...

To see this, we turn to the most excellent Umberto Eco (and it is worth reading in full):

What I mean is that to tell a story you must first of all construct a world, furnished as much as possible, down to the slightest details. If I were to construct a river, I would need two banks; and if on the left bank I put a fisherman, and if I were to give this fishermana wrathful character and a police record, then I could start writing, translating into words everything that would inevitably happen. What does a fisherman do? He fishes (and thence a whole sequence of actions, more or less obligatory). And then what happens? Either the fish are biting or they are not. If they bite, the fisherman catches and then goes home happy. End of story. If there are no fish, since he is a wrathful type he will perhaps become angry. Perhaps he will break his fishing rod. This is not much; still, it is already a sketch. But there is an Indian proverb that goes, "Sit on the bank of a river and wait; your enemy's corpse will soon float by." And what if were a corpse were to come down the stream – since this possibility is inherent in an intertextual area like a river? We must also bear in mind that my fisherman has a police record. Will he want to risk trouble? What will he do? Will he run away and pretend not to have seen the corpse? Will he feel vulnerable, because this, after all, is the corpse of the man he hated? Wrathful as he is, will he fly into a rage because he was not able to wreak personally his longed-for vengeance? As you see, as soon as one's invented world has been furnished just a little, there is already the beginning of a story.

Eco, U. (1994). The novel as cosmological event. In The Name of the Rose (1st ed., pp. 512-515). San Diego: Harcourt Brace.

Exactly. Of course, if you're creating a persona you're working off of data, and the construction of the world is not random but is informed by your data (though there may be a few judgement calls along the way for clarity's sake, or to include perspectives). Importantly, the more constraints (user stories, situations) you add, the more formed your story (persona) gets.

Eco continues:

The problem is to construct the world: the words will practically come on their own. Rem tene, verba sequentur: grasp the subject, and the words will follow

Indeed.