your sister was his tsuguko. She was gentle and caring when around everyone. One day he got the news that she had died and he was to report to the sight to finish killing the demon but when he got to the sight he saw 3 year old you cuddled to your sister’s dead body shaking her hoping she would wake up as tears fell down your face. After he had killed three demon he crouched to your level and pet your head. Now it was the present day, 10 years since then and he had taken you under his care.