On Princess Principal's ending
(There are some spoilers here.)
In my retrospective on the summer season, I said that Princess Principal wound up as more of a prequel than a story and waved my hands a bit about why that was so. Today I want to write more details about that. The first question to ask is if Princess Principal has a conventional ending. Usefully I can answer that based purely on story structure, without having to talk about the specifics of what happened.
There are two ways to have a conventional ending to a story; you can resolve a significant ongoing plot issue or you can move some dramatic characters significantly forward in their character arcs (or you can do both). If you're ending an entire work you wrap up the big things (for characters and/or the plot); if you're just ending a season, you just wrap up a medium-scale plot or move characters forward but not all the way to the end of their stories. Looked at purely through this structural lens, Princess Principal does neither. The show had no large scale plot as such (although it did have an overall situation that created the fundamental story conflict), and while the protagonists were all dramatic characters, none of them resolved their character arcs or ostensibly made dramatic changes.
At the same time, things very clearly happened over the course of Princess Principal; the protagonists all wound up in a significantly different place than they started out. The show is not simply an episodic collection of adventures where at most we find out character backgrounds and then get a brief two-episode 'climax' at the end. That these changes happen and what they are is why I call Princess Principal a prequel.
In the very first aired episode of Princess Principal, Dorothy and Chise have what is in retrospect a crucial conversation after Dorothy casually lies to some normal students:
Chise: That was a bold move.
Dorothy: It's best not to sneak around with these things.
Chise: I see Ange isn't the only one with a knack for lying.
Dorothy: Spies are all liars. You're lying to me now, Chise.
Chise: As are you.
Dorothy: So what do you say we try being honest with each other?
Chise: The idea has its charm. But if we stopped lying, we wouldn't be able to stay friends.
Dorothy: Is that really friendship?
Chise: Even parents and children lie to each other.
This deliberately sets up the usual genre atmosphere for spy stories where all of the characters have their own interests, trust is purely temporary, things aren't as they seem, and betrayal may lurk around the corner at any time. All of the protagonists have their characteristic roles in this atmosphere; Ange, Dorothy, and Chise are outright spies with their own interests and secrets, Princess is the mystery, and Beatrice is the naif outsider. In fact the entire first episode is there partly to establish this overall atmosphere, since the episode's plot is a classical spy story of deception, hidden motives, and betrayal.
Over the course of the rest of the series, all of that changes. As all of the protagonists undergo character development, we see them quietly transmute from a collection of spies thrown together into a group of comrades. This reaches its climax in the ending of the show, where over the course of the show's only real plot arc, one by one all of the characters deliberately choose to turn their backs on their previous associations and instead choose the people who've now become their friends. By the ending epilogue, these people have stopped being a group of spies thrown together and become a team that happens to work as spies. It's to Princess Principal's credit that all of these decisions feel inevitable in the light of everything we've seen the characters go through together. Of course Chise is going to come back. Of course Dorothy is ultimately going to quietly choose the people who've become her friends, and to let Ange know that.
In other words, Princess Principal is the origin story of a team, the prequel that explains how they came to be before they go on to have thrilling adventures together (if Princess Principal ever gets another season). It's not a whole story in and of itself, because it doesn't really go anywhere or resolve anything (either in plot or in character development), but the characters themselves change in important ways; they end as different people than they started and they've made real decisions in the process.