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chapters 63, 78, 105 and 121.
and each time, Atsushi's reaction was to acknowledge Dazai's presence and be motivated by it:
but, now that the veil has been lifted, it's very easy to see that this Dazai hasn't been very nice in his approach at all. Here, let me help you visualise it a little:
Looking back, that Dazai's words were systematically accusatory and harsh, and if at the start his face resembled the man Atsushi knows, recently his expressions have turned dull and cold, much more reminiscent of what we've seen from the orphanage headmaster.
Yes, the headmaster was cruel. Yes, it was unfair. No, Atsushi didn't deserve any of it. He should have been protected, he has every right to despise that man and to feel all the conflicting feelings he has about it. But it happened. It happened, and he was shaped by it, and every step he takes as he makes his way through his new life has the echoes of the way that man raised him. He's making the choice not to let anyone go through what he has to if he can help it, but also makes him prone to self-sacrifice because of his lack of self-esteem.
Atsushi's been struggling hard since the Decay of Angels arc began, and the voice that's been motivating him whenever his determination waned has been the one of the man who raised and abused him hidden behind the mask of the one who brought him to his new home. It's fucked, and it worked.