#bsd q

Timeline wise, it fits. Q mentions Dazai's 'new friends' are easy to break, which implies the person Q targeted in the past was likely someone Dazai was generally close with. This could mean;

Odasaku, Ango, Akutagawa, Chuuya (I know some of you are going "They aren't friends!" but their relationship is complicated so we'll consider them as people Q would have thought were Dazai's friends.)

Q's ability has conditions, it's a single target ability that activates when Q is injured and the doll rips. Ango doesn't have a combat ability. Oda is incredibly efficient in combat but his proficiency is in guns and martial arts. Akutagawa is a very likely contender but the reason I think it's Chuuya;

Chuuya considers Port Mafia family and values life. If he were the one to be affected, the one that murdered his companions, I think he would be the most distraught. And Chuuya's destructive force is unparalleled even without corruption. His use of combat combined with gravity makes him absolutely terrifying.

On top of that, Chuuya's humanity is something that is important to him and on some level, Dazai as well. Dazai was willing to give up a plan with high chance of success to save Yokohama and try to quickly come up with another if it meant Chuuya could have answers to what he is. Chuuya sacrificed that and went with the plan with the highest chance of defeating Verlaine.

Dazai also mentions hating mind control, again, something that's particularly important in Chuuya's case thanks to the coding in Chuuya's head possibly forcing him to activate corruption against his will, to be used the way Paul Verlaine was with forced command orders. So Q taking away Chuuya's autonomy and breaking his spirit would be a good reason, imo, why Dazai does not like Q.

Референсы из Гангсты. Подпольный врач, его помощница и два их местных друга бандитского вида)