What is tsuguko?

A Tsuguko (継子) is a successor designate in the Demon Slayer Corps. They are hand-picked by a Hashira to be their apprentice and eventually, potentially, their replacement.

Here's what's important to know about them:

Role and Purpose: Tsuguko are essentially trainees groomed to become Hashira. They learn the Hashira's skills, techniques, and philosophies, preparing them for leadership within the Demon Slayer Corps. Their primary purpose is to become strong enough to eventually take the place of their Hashira, should the need arise.
Selection: Hashira carefully choose their Tsuguko based on potential, compatibility, and dedication. There's no fixed age or rank requirement, but they're usually promising Demon Slayers. It's a personal decision made by the Hashira.
Training: Tsuguko undergo rigorous training under the guidance of their Hashira. This training includes honing their swordsmanship, mastering their Breath Style, and learning to strategize against demons.
Relationship with Hashira: The relationship between a Hashira and their Tsuguko is one of mentorship and guidance. The Hashira invests significant time and effort in shaping their successor.
Succession: While the ultimate goal is for the Tsuguko to succeed the Hashira, it's not guaranteed. A Tsuguko must prove themselves worthy and capable of taking on the responsibilities of a Hashira. Sometimes, a Tsuguko may not reach the required level of strength, or the Hashira might die before the Tsuguko is ready.
Notable Examples: In the series, Kanao Tsuyuri is the Tsuguko of Shinobu Kocho, the Insect Hashira.

In short, a Tsuguko is a vital part of the Demon Slayer Corps structure, ensuring the continuation of powerful and experienced Demon Slayers to protect humanity.