Demon Slayer's nine Hashira are humanity's last line of defense against the vicious demons that rule the night. It's not an easy job, and only the best of the best can take on this prestigious mantle. As such, the only way to become a Hashira is to kill at least 50 demons or defeat one of Muzan's overpowered Demon Moons.
Source：https://www.cbr.com/demon-slayer-saddest-hashira-fates-ranked/
