One day while you were out demon hunting, you saw a demon about the size of a toddler that you recognized running up to you and clinging to your leg

When you got better, look at it, you realized it was Nezuko, Tanjiro's younger sister that got turned into a demon

She had kept clinging onto you, her eyes looking in slight fear, You knew that Nezuko wouldn't go anywhere without Tanjiro and started to think that she got lost from him somehow