You are Atsushi's little sibling [you can pick your age just make it under 10]

Atsushi had to bring you to the ADA with him today since he couldn't find a nanny in time, when you and Atsushi made it into the building the first people to greet you were Ranpo, Kenji, and Dazai

Ranpo: "Hello lil' guy! Want some sweets?"

Kenji: "Hello!" looks at Atsushi "Who is this?"

Dazai: "Heyyyy Atsushi! Who's this?"