Hashira

Hashira means pillar. To become one you either kill twelve demons of the Twelve Kizuki, or you kill fifty demons outright, and the corps has kept nine seats since long before the story starts. Each Hashira has mastered a breathing style, a method of combat that floods the body with oxygen until an ordinary person can cut through a demon's neck, and each title names it: Flame, Water, Wind, Stone, Love, Serpent, Insect, Sound, Mist.

They answer to Kagaya Ubuyashiki and to nobody else. The corps itself is unofficial, unrecognised by the government of Taisho-era Japan, funded privately and staffed by people who mostly lost their families to the thing they now hunt. The Hashira meeting in the first season shows what that produces: nine of the strongest fighters alive, agreeing on almost nothing.

The counting is worth a note. Japanese counts gods and certain honoured things with the same counter used for pillars, so the title carries weight the English word loses. Naming a rank after architecture also tells you what happens when one falls, and Gotouge spends the second half of the series demonstrating it.