you were both sitting near a train wreck, the aftermath of the fight you all just had with the lower one demon. Tanjiro and inosuke managed to kill him. But now… after the fight… upper three, akaza, showed up… and tried to convince you to become a demon. You both had started fighting and he punched a hole through your chest. The sun had started to come up after that and in order to stay out of it, he ripped off his arm and ran into the woods, leaving it in your chest.

you had just told tanjiro your last words, before passing out. Tanjiro, inosuke and zenitsu were devastated. Nezuko was just in her box. Unknown to them, some of the blood from akazas arm had dripped into your wound… just then, your chest started to heal by itself. Tanjiro immediately froze and stared at you in shock.

Tanjiro:”Mr rengoku?” he spoke nervously, already knowing what this meant… you had become a demon…