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I feel this is even more apparent in Beast, seeing how they act under different circumstances. While they were affected by their pasts in the same way, seeing how, for example, Atsushi's whole bit about how he kills because he's afraid of death. It shows how fearful he is and how far he's willing to go in the name of self-preservation, even though he acts somewhat more 'softer' than Akutagawa in both universes. Or how he's trying to 'escape' the guilt of killing the Headmaster.
Or when Akutagawa gets mad at Junichiro for being overly rough with Naomi in his entrance exam. Not only does that show how he's a 'protector at heart,' but it shows how kind and caring he is on the inside, even if he has an odd way of showing it. Another example with Akutagawa is his unlikely friendship with Kenji and how careful he is with farming. It may be small, but it shows he gentle Akutagawa is capable of being, despite it all.
Sorry, that post just got me thinking lol
OK correct me if I'm wrong, but I feel like the main 'yin/yang' parallel with Atsushi and Akutagawa is not something like 'this one is bad but secretly has a good side and this one is good but secretly has a bad side'.
I feel like it's more about 'who they are at their core vs who they choose to be'.
At his core Akutagawa is kind and at his core Atsushi is not. But despite this Atsushi tries every day to make the kinder choices and I love him so much for it. He has to work so hard to be good.
He wants to be a bitch SO bad I know he does but he tries his best to help people and be nice (sometimes he fails but that's OK <3)
Atsushi doesn't always WANT to help people, a lot of the time he's selfish and scared, but he does help people anyway. He keeps helping people over and over again. There's still some selfish motivation to it, and his initial motivation for helping people was because the headmaster told him that's all he was worth, but overall he does care about the people he helps and it weighs on him if he fails to save them. And of course, as the series goes on he starts helping people more because he can rather than because he feels like he needs to.
In Akutagawa's case, he's still capable of being kind but his environment led him into being someone who chooses to hurt people. But he's always been a protector at heart. In the start he was bad compared to Atsushi because he was choosing to hurt people and keep the cycle of abuse going. Just like how Atsushi developed in why he saved people, Akutagawa starts to get redeemed when he chooses to not just act on his rage. Not only does he start to spare people, but he speaks more kindly to them (apologising to Higuchi and telling Kyouka he's proud of her). It all culminates into the moment he chooses to help Atsushi and sacrifice himself for him, going back to his core value of being a protector. Even when he's finally revived, he keeps this role in his new position as Aya's Knight.
I kind of see the streaks of white in Akutagawa and the streaks of black in Atsushi not as their 'hidden sides' but as their fundamental selfs. That's who they are at their core, and their main colours (black for Akutagawa and white for Atsushi) are how they're presented to everyone else and how they try to have people see them as.