Kyojuro
Rengoku

Rengoku is the Flame Hashira, and for most of the audience he is the cost of the Mugen Train made human. His mother Ruka told him on her deathbed that the strong exist to protect the weak, and he built his entire life on that sentence. His father, a former Flame Hashira, collapsed into drink and told his son he was wasted effort. Kyojuro trained anyway, alone, from his family's instruction books.

On the Mugen Train he backed up his creed in full view: two hundred passengers, and not one died. After Enmu's defeat, Upper Three arrived, and Rengoku fought Akaza to a standstill as a human against a demon with a century of accumulated strength. Akaza spent the whole fight begging him to become a demon and keep his talent forever. Rengoku refused with a hole through his chest, held Akaza in place until the sun forced him to flee, and died at dawn passing his last instructions to Tanjiro.

"Set your heart ablaze" outlived him. Tanjiro carries it, Senjuro his brother took it up, and even his broken father was moved to write something like an apology. He appears in ten episodes and shadows every arc after.