You're Atsushi's younger sibling. You and Atsushi were not aware of each other until Atsushi got kicked out of his orphanage and went on a search to find his biological family after becoming a detective

The only family member Atsushi was able to track down was you, his younger sibling. You were an orphan like him so Atsushi became your guardian

Right now you're walking home from school but then you see Atsushi on a bridge investigating a crime scene