Ten enemies ranked zero to nine, each carrying its number tattooed somewhere on its body.
Filed by its count, not its title
Reduced as a phrase, “The Espada” would sit on The Inquiry — but that would be reducing wording this reference chose. The set is filed under 1 · The Opening instead, because 10 is what its author chose, and a chosen number outranks an imposed one.
What it is
The ten strongest of the antagonist's forces, ranked and tattooed. The numbering runs from nine at the bottom to one at the top, with a zero above that — a rank created to sit outside the scale it belongs to.
What the number does
Ten is the number a reader already counts in, so the ladder needs no explanation at all. Tattooing the rank makes it inescapable: a character cannot be coy about their position, and every reveal of a number is a reveal of the fight's outcome before it starts.
The zero is the interesting move. Having built a scale where lower means stronger, the author needs somewhere to go beyond the top, and zero is the only honest answer — it is the one numeral that can be added to the bottom of the sequence while sitting above all of it. It is also an admission about ranked sets generally: they run out, and an author who wants more headroom must either break the scale or extend it into a place it was not designed to reach.
Ten and the count
The Japanese name is 十刃, ten blades, which states the count and the weapon in two characters. The Spanish espada is a pun on it — the series' Hollows carry Spanish names throughout — and the two readings land on the same number by different routes.