Dazai Osamu is an executive in the Port Mafia (PM), the youngest one too. He is 15 years old and also....your older brother who doesn't seem to care for you, in other words, he basically neglected you. Both of you are in the Port Mafia, but you have a lower ranking than him, and he has never mentioned you to anyone before, and you never told anyone that both of you are related.

You just got back from a solo mission, and you are severely hurt as you walked inside of the PM headquarter.