The Flags' views of/relationships with Dazai headcanons:
Piano Man: No strong personal feelings. Was inclined to like Dazai because he's a Mori loyalist but finds his demeanor and (lack of) values too off-putting. He sometimes worries that Dazai will beat him to the executive seat but would never admit it. Also he thinks that Dazai and Chuuya's rivalry is funny.
Albatross: Didn't think much of him at first, but then Dazai's obsession with Chuuya triggered a one-sided beef that made Dazai scheme against Albatross. So then it turned into a two-sided beef because normally Albatross would kill anyone who messed with him like that but Dazai is off limits. Albatross and Chuuya bond over hating on Dazai but once Chuuya gets going even Albatross will have to cut him off like "hey man it's not that serious can we talk about something else?"
Doc: Whenever Dazai comes on his radar Doc finds him amusing. Dazai usually gets treated by Mori but sometimes he'll end up in the regular clinic. They have philosophical discussions where Dazai will say something self-deprecating like "I don't qualify as a human" and Doc will assure him that he is definitely still a human being, and that the only reason he's depressed is because God has chosen to abandon him specifically.
Lippmann: Lippmann is polite to Dazai in the way that you would expect a celebrity to be polite to a kid with cancer. They both specialize in mind games and enjoy a bit of verbal sparring. Dazai tries to bribe Lippmann into letting him in on his various schemes to dress up Chuuya or otherwise rope him into his industry, but Lippmann always declines because he doesn't want his playtime with Chuuya tainted by Dazai's untoward intentions.
Iceman: Dazai once came to him for advice on how to kill Chuuya and Iceman trolled him with his answer. That was about it.