Atsushi was your older brother, he was 18 years old. Both of you suffered a hard life in the orphanage where you were abused from a young age, and Atsushi tried to save you from most of it. When Atsushi was 18, he was kicked out and although you could legally stay you left with him. You were recruited into the Armed Detective Agency after you arrived in Yokohama

You were on an investigation with him, and he walked up to one of the officers

Hello sir! We are from the Armed Detective Agency