Chuuya didn't know how or why he was born. Where? What was happening then? He didn't know that. He didn't even know his parents, it wasn't given to him. As a child in the lab, at the age of 7 he merged with Arahabaki and lost his memories of everything before. Then the slums, Sheep, the beginnings in Port Mafia and the Verlaine thing, Dazai leaving PM and went to ADA. He didn't even have time to think about where he was coming from.
One day, however, he decided to try. As an executive of Port Mafia, he had access to some files, and Mori, as a boss and doctor, helped find the sources. It took several months, and the search was ongoing. Finally, Mori brought him the files. He did a genetic test on all the residents of Yokohama. Mori had a plug into the hospital's data, so they compared results for his DNA.
Chuuya spent the next few days researching as much as he could about you. Where do you live, who are you, what do you do. You had an 8% match. You were his relative. He finally found his family. He was scared and excited at the same time, which was rare.
And so it happened. He found your address and went to it. He took a deep breath and rang the doorbell. When you opened the door, he was practically choking with nerves.
That day he told you everything and showed you the evidence. You were shocked that he had records of hospitals in Yokohama, but the results didn't lie. You were distant family.
Since then he has been visiting you, wanting to get to know you as well as possible. Sometimes he comes for fun, sometimes out of boredom, sometimes even to complain about a few wounds or for some comfort. He trusted you without knowing you, and he would never let his guard down like that in Port Mafia.
Today you were sitting on the couch watching something on TV until you heard the doorbell. You got up and opened the door to find Chuuya alone, still in the outfit he wears while "working" at Port. He looked at you with a familiar smile. He saw in you what he had wanted for so long... a family.