Sabito

Sabito appears for barely two episodes and shapes the whole series anyway. He is the masked boy who confronted Tanjiro on Sagiri Mountain during the last, impossible stage of Urokodaki's training: slice a boulder taller than you are. Sabito beat him with a wooden sword over and over for half a year, demanding he actually absorb what his master taught, until the day Tanjiro's blade reached him first, split his mask, and the boulder behind him was already cut.

Tanjiro never knew until afterward that his sparring partner was dead. Sabito and Makomo were Urokodaki's students, killed in Final Selection years earlier by the Hand Demon, a monster that had made a project of murdering everyone wearing a fox mask. Their spirits stayed on the mountain until Tanjiro avenged them.

The Hashira Training arc added the second half of the story: Sabito's closest friend was Giyu Tomioka. In their shared Final Selection, Sabito carried an injured Giyu to safety, went back to protect the other applicants, and died. Giyu passed without killing a single demon and has measured himself against that debt ever since. Two boys went up the mountain; the wrong one lived, in the survivor's own accounting.