Maybe one day we'll know for sure... but this is my idea for what went down and the person I think Q targeted for maximum chaos and damage
Reasons why I think it could be Chuuya;
SPOILERS FOR STORM BRINGER
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.
Again, all speculative :) Hoping one day we'll see what happened. Honestly I'd love to see more of Q.
Just read the new bsd chapter and omg Fyodor using a quote from irl Atsushi Nakajima and Atsushi recognising it is so interesting omg we're getting Atsushi and Fyodor lore!!!
The quote in question is "The head may err but never the blood" which before Fyodor said that he said "I'm relieved. You truly are the bookmark, you are still stuck on those words."
What the hell is that supposed to mean?
WAIT, apparently the meaning of this quote is that your mind may mislead you but your instinct's never will. I had this theory that Atsushi could just simply locate the book by believing that he can do it and I think this is just confirmation of it.
I'm aware we don't actually know where abilitues come from (or, like, 90% of the magic system in BSD, for that matter) aside from the hereditary stuff presented through Kyouka or Tachihara and his brother; but there are some devastating implications to the way Atsushi's ability manifested for the first time (as told in Dead Apple).
Beast Under the Moonlight came from his desperate need for survival and having no other way to protect himself. So, a breaking point. A devastating one at that. What was the breaking point for other characters like?
Sigma's main emotional wound comes from having no memories, no knowledge of his past, of his existence, no place to belong. And his ability is to exchange information through touch. Sure, this might have been written in on the book alongside everything else about him, but there is still a poetic element to it.
Or, what drove Kyusaku to manifesting an ability that only works on people who hurt them? A child who has the ability to drive those who hurt them insane. What happened to them?
What was Dazai doing before his attempt that made him end up being able to nullify other abilities, essentially making him impossible to attack via non-traditional means?
How desperate was Fukuzawa for human connection while dealing with his trauma from the war that he manifested an ability that works by bringing people together?
How much love for life does Yosano carry that her ability is to heal and revive people with essentially no limit to it?
How strong is Kunikida's resolve to bring his ideals to life, no matter how impossible they might be in practice, that he can bring objects to reality from a blank page?
What did Oda want to prevent? What did Ango want to find out?
I could (and do) think about these characters all day long ahhhhhhg
This confirms that Dazai is literally the only person who can kill Fyodor since he would nullify the possession. Which makes the "You can't kill me line" way funnier because Fyodor was really thinking
"please for the love of god do not be the one to kill me"
Atsushi being the PM transfer could actually be the funniest thing ever. Like everyone thinks it's gonna be Dazai. The audience thinks it's gonna be Dazai, fukuzawas flashbacks think it's gonna be Dazai, hell, Dazai himself probably thinks it's gonna be Dazai.
So imagine when the time comes for the choice to be made, everyone's lined up and Mori's about to make his decision. Dazai starts to walk forward only for Mori to point at atsushi and go, "I want the weretiger"
Like imagine the SHOCK. because why in God's chaotic earth would he pick atsushi? 😭
(Turns out Atsushi was pretty impressive during the fight with Fyodor, and Mori reallyyy wants his soukoku 2.0, but I'm sure he probably has many other reasons hidden in his rich little sleeve)
So if BSD is going to conclude Part One with Ch.130, does that mean Fyodor's backstory will be revealed in the second part?? Will the Order of the Clock Tower make their debut?? Is Naomi's past going to be relevant later in the story?? Will we see Paul Verlaine more now that he is out of the basement?? Also who will get transferred to Port Mafia from ADA?? If Atsushi gets transferred, will it be Beast 2.0?? Will Chuuya and Atsushi finally interact??
I have so many questions. Atleast we are getting out of that fucking airport for now