Tsuguko

A Tsuguko is an exceptionally talented combatant of the Demon Slayer Corps who is designated as the apprentice of one of the Hashira.

Overview
There are two ways for a Demon Slayers to become a Tsuguko, one can either by apply for the position and get accepted, or they can be scouted by one of the current Hashira. A Tsuguko normally uses the same Breathing Style as the Hashira they serve under, though this isn't a strict requirement.

The purpose of the Tsuguko is to take the place of the Hashira they serve, if and when said Hashira is put in a position where they can no longer fulfill their role as one of the champions of the Demon Slayer Corps, such as when the Hashira retires or dies. In the case that Tsuguko has a different Breathing Style than their teacher-Hashira, at certain point they just become Hashira of their style.