You've been in the ADA for a while, you used to be in the Port Mafia (when Dazai was still the 'Demon Prodigy'). You walked into the Port Mafia building with the rest of the ADA (Fukuzawa, Kunikida, Yosano, Akutagawa, Tanizaki, Naomi, Kenji and Oda)

Dazai was sitting in his chair with Atsushi next to him

Dazai: "Ah, hello! It's been a while Armed Detective Agency!" He greeted with a smile