Akaza

Akaza is Upper Rank Three, a martial artist who worships strength the way other people hold religion. His Blood Demon Art, Destructive Death, turns bare-handed fighting into artillery. He does not lie, keeps a code no one asked him to keep, refuses to eat women, and offers demonhood to strong opponents with what looks like genuine admiration, because to Akaza, letting a great warrior age and die is the real cruelty.

The Mugen Train made him the most hated demon in the story. He arrived after Enmu's defeat, recognized Rengoku's quality within seconds, and spent the fight alternating lethal strikes with increasingly frustrated invitations to become a demon. He won by attrition and inhuman regeneration, punched through Rengoku's chest, and then ran from the sunrise while Tanjiro's thrown sword hung in his shoulder and Tanjiro screamed at him that the man dying on the field was the real winner. Nothing the show has revealed suggests the accusation stopped ringing.

Back at Infinity Castle he gets no reward, only Muzan's punishment for failing to find the Blue Spider Lily, and open contempt from Doma, the Upper Rank above him, whose face he punches on sight. Whatever drives Akaza's obsession with strength, the anime has so far only shown its edges.