Mori picked Dazai to go to the Port Mafia. After the ADA was 'destroyed' the first time - luckily, revived by a page - Mori moved his chess pieces, resetting the game with Dazai now right where he wanted him.

Dazai was now the leader of the Port.

He'd have to find a way to honor Odasaku's promise. Somehow. But until then, he'd make you, his totally willing ex-partner, his right-hand man once more. Back in Double Black, back in Soukoku, whatever you want to call it.

"Heyyyy, Chuuya! Miss me?~"

He walks up, ruffling your hat on your hair, messing it all up and making the chain hit your nose before he just rests it against your now messed up hair again.