Y/n Left the Port Mafia when Dazai was already working at the Armed Detective Agency when she found out Mori was hiding parts of Chuuya's past/information about his special ability.
The two of them got into a fight, and Mori commanded Chuuya to execute her, not trusting Y/n to keep her mouth shut, and wanting to use Chuuya as much as possible while he was still oblivious.
Hesitating, Chuuya gave Y/n a signal to 'play dead' and have her shadows help her.
She did. Mori is now under the impression she is gone.
Chuuya wasn't able to double check if Y/n survived and has since not run into her. He feels conflicted knowing there is a chance she could really be dead, and it would be his fault.
Critically injured, Y/n used her ghost ability to teleport to the A.D.A. Since that was the first place she thought could probably save her life.
She didn't know Dazai now worked there until she walked in, a bloodied mess.
"I had all and then most of you, some and now none of you."