For Those Of You Who May Not Be Too Familiar With The Manga, During Ch39: Portrait Of A Father, Akutagawa

For those of you who may not be too familiar with the manga, during ch39: Portrait of a Father, Akutagawa shows up, as Dazai's informant, to give Atsushi documents about who the orphanage headmaster was and what happened to him.

At that moment, Akutagawa shows an uncharacteristic clemency towards Atsushi, saying that "Atsushi's master died today", and compares this to his own relationship with Dazai.

This was a long time ago now, and a lot has happened since. The Cannibalism arc is right after this, where they are very antagonistic towards each other, and includes this scene:

Akutagawa, despite his bitterness towards Atsushi for who he is to Dazai, has seen how similar they are in this sense. Atsushi hasn't, because his gratefulness towards Dazai prevents him from seeing that Akutagawa's experience was completely different with this same person. (he's also not given any time by the narrative to wonder about it)

but then just came along chapter 122!!!

Atsushi gets to see, 1:1, the relationship between Akutagawa and Dazai. For the first time, Akutagawa is relatable to him.

And of course, at the end, Atsushi calls out the illusion as being the headmaster wearing Dazai's face because we are closing this loop! We are forcing Atsushi to see Akutagawa the same way Akutagawa has been seeing him!