-I believe that Doc is a runaway. His parents weren't abusive in any way but after they were told about his medical condition they would not stop guilt tripping him, shaming him, telling him he shouldn't show himself to other ppl ect. At some point he got sick of their whining and ran away to study medicine so he can fix his condition on his own but along the way he met Mori and was dragged into the war because of his impressive knowledge on media stuff and ability.

-in the light novel its said that one of the flags has a quick sand ability and i believe its Doc's. And following that thought I'd like to think that during his time in the war even tho he mostly specialised in the medical field he was occasionally sent to the battle field to help out, although he only used his ability at a safe distance so he wouldn't get hurt bc he's too important for Mori

-Yosano's main "role" in the medical staff was to heal the soldiers that were near dead or missing limbs ect since she didnt have any real medical knowledgeat at the time. Doc(and Mori) were taking care of soldiers who had non-lethal injuries but still needed to be treated so they don't BECOME lethal and/or were preventing the soldier from properly doing their job

-during the "dark times" of the war(when the soldies were losing their minds) similar to Yosano, they would often attack Doc while he was trying to treat them. Because even then Doc was quite weak from his sickness he was a bigger target and got injured more easily then the other medical staff, at some point there had to be someone else in the room while Doc was treating the soldiers. They would also sometimes leave him behind at the battlefield while he was helping with his ability, so he also visited Yosano once or twice before she was sent to the asylum

-after the war Doc lost contact with Yosano and worked as an assistant/subordinate to Mori until Piano Man was sent to him to treat an injury he got on a mission(or from the old boss). They got along really quickly over their shitty bosses and Piano recommended that Doc should become his subordinate and he wouldn't have to deal with Mori any more. That hope was soon shattered when he became boss though.

-during the first few months after becoming the agency's official doctor, Yosano would have moments where she's confused on why an agency member is coming to her for help with a non-lethal wound since she's used to leaving that job to Doc

-Yosano never learned about Doc's death not even in a Ada Chuuya au bc Chuuya doesn't have the heart to tell her

-in another universe where the flags lived, Dazai BEGS Chuuya to bring the flags with him during the next Ada and Pm meetings (because he wants to bother Piano Man like old times) and Doc and Yosano have a cute lil reunion and they stay in touch after that

These are the context hc's about Piano Man's relationship with Dazai and the old boss :