Thirteen numbered squads, each with a captain — the largest numbered set in mainstream shōnen, and the one that most nearly buries its author.
Filed by its count, not its title
Reduced as a phrase, “The Gotei 13” would sit on The Ledger — but that would be reducing wording this reference chose. The set is filed under 4 · The Structure instead, because 13 is what its author chose, and a chosen number outranks an imposed one.
The military of the Soul Society, organised into thirteen numbered divisions, each led by a captain and a lieutenant. The number is in the name.
Thirteen is an institution rather than a roster. At this size the reader stops tracking individuals and starts tracking an organisation, which is exactly what the story wants — the squads exist so that a bureaucracy can be the setting, with captains who outrank each other on paper while being wildly unequal in practice.
The cost is severe and instructive. Thirteen captains plus thirteen lieutenants is twenty-six named characters, each of whom the reader reasonably expects to see fight. No serialisation can pay that debt evenly, and the series' long middle is in large part the sound of an author servicing a number he committed to early.
Thirteen carries no unluckiness in Japan — that association is Western, imported and largely decorative here. What it does carry is oddness: a prime, unsplittable into the tidy halves that let a twelve be managed. The organisation cannot be tidied, and reads as older and more administrative for it.