you were 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 started running into the woods, leaving it in your chest, before he paused. By doing what he just did, some of the blood from his arm would drip into your body, and turn you into a demon... if he just left you there, the sun would certainly kill you!

you were just standing there, unable to move because of the demons blood. you then felt akaza grab your hand, and before you knew it, you were running into the woods with him.