Okay so this isn't something canon or etc etc but just a thought that developed in my sleep deprived brain.
So we all know that beast is a universe which is created by dazai himself. So more or less everything he wanted in the canon universe is there .
We can see that he created this universe to save oda by putting him in the ADA . He himself became the mafia boss and crept into even more darkness just to save his friend in one alternate universe and we can see how happy he is when he finally meets him. He even goes as far as sacrificing himself just for everyone's happiness.
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So this oda thing was something that everyone knows. Now we come to shin soukoku. Akutagawa who never saw the world of light was given a chance to live there . Though he captured gin and all but i guess that was all a plan of his to meet oda. Also we can see that their is a much friendly bond between atsushi and aku in this . The cafe scene in which they talk about their childhood ( which is so similar ) literally gives a new point of view to their bond. But since no one can be entirely happy according to asagiri sensei so in this au they have to suffer about different things like atsushi traumatised about killing his father figure and aku and gin's whole scenario. Thus, we can conclude that dazai gave a chance to aku in beast au to live in the world of light by putting atsushi in the mafia.
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So mostly all the main characters were switched , dazai clearly knew what he was doing by switching the roles of him and oda , atsushi and akutagawa. Now let me come to the main point i.e soukoku . He didn't swap chuuya in the ada. Like he knew how he was more human than anyone else , how he valued life but still he decided to put him in the mafia and that too as his personal assistant/ bodygaurd / dog whatever ( i mean no one is allowed in boss's office without permission but we can see that he's 24/7 beside dazai ) . So in this au he is keeping chuuya by his side who gives him life i mean the will to live. In the darkness chuuya is the only one who shows him a bit of light and before whom he can act like a real teenager. We can see how he acts like a kid in front of him . Hence it's true that wether it be canon universe or an alternate universe chuuya will always be beside dazai , supporting him, bickering with him , giving him death threats, but caring about him all the time which in terms of their bond is called being partners.
