Akutagawa isn't Yin, and Atsushi isn't Yang (or atleast not completely, Part 1: Symbolism)
Everybody and their mother knows that Shin Soukoku is supposed to represent Yin and Yang. Judging by their designs and personalities, you’d think it would be easy to tell which is which.
Just as a reminder, because I know a lot of people get this confused, the black side with the white dot is Yin, and the white side with the black dot is Yang.
Now, purely judging by their designs, you’d assume Atsushi is Yang and Akutagawa is Yin. Atsushi’s hair is white with a singular black streak, and his outfit is white with black detailing. Meanwhile, Akutagawa wears a black coat that hides a white blouse (or shirt? I’m not really sure what he’s wearing; why Gin, fashion icon that she is, allows her brother to leave the house looking like that, I don’t know, but I digress). He also has black hair with white tips. So, it's black on white and white on black.
Even their personalities match up! Atsushi is the "sunshine boy" with a hidden darkness inside. His desire to save people is deeply selfish at its root—he does it to prove his own right to exist, not out of pure, selfless altruism. Akutagawa, despite being a cold-blooded Mafia executioner, possesses a fierce, unyielding code of honor and loyalty. He spent his youth protecting orphaned kids in the slums even though, logically, it would have been better for him to let them shrivel up and die. And every day in the manga, he gets better and better.
So, you might be wondering, "Shamangus! You just spent the whole beginning of this post telling us that Akutagawa represents Yin and Atsushi represents Yang—is this some sort of clickbait!?"
The thing is, the interpretation that Atsushi is Yang and Akutagawa is Yin isn't wrong at all; I’m just saying it can be the other way around. In fact, more often than not, it actually is the other way around.
The first part of this analysis focuses on symbolism. We’ve touched on the seeds, but I want to talk about the Tiger and the Dragon.
The Dragon and the Tiger have long been used to embody the ultimate cosmic balance of Yin and Yang in East Asian philosophy. While this concept originated in China, China is a massive cultural influence on Japan, where these ideas were exported. (And even if they weren’t, the real-life Atsushi Nakajima’s affinity for China makes this a topic worth talking about.)
Obviously, Atsushi represents the tiger; I don’t think I even have to elaborate on that. He can turn into a massive "fuck you" tiger. But based on what we just said, you’d expect the tiger to represent Yang, right? Nope. The tiger is actually the Yin in this specific symbolism.
Now, take the dragon. First of all, Akutagawa’s first name literally means "son of the dragon." Also, Rashomon kind of looks like one.
Rashomon resembles a traditional Chinese dragon, and the real-life Ryūnosuke Akutagawa was born in the exact year, month, day, and hour of the Dragon in the traditional Chinese zodiac. So yes, he is very much "dragon-coded," and the dragon represents Yang in this framework.
Next, we have the Sun and the Moon. The Sun represents Yang, and the Moon is Yin. Your natural assumption is that Atsushi is the Sun and Akutagawa is the Moon. But I have to point out that while the Port Mafia does rule the night, the ADA doesn’t actually work during the day; they work during the "twilight"—whatever that means.
Not to mention, the first time we see Akutagawa, it is in broad, sunny daylight while he’s blowing up a police station.
Atsushi, meanwhile, has an ability literally called "Beast Beneath the Moonlight." Do I need to get any more serious about this? When he meets Akutagawa, it is sunset, which quickly transitions into the night. While I don’t have a ton of "evidence" that Akutagawa is particularly sun-coded, I think it’s pretty obvious that Atsushi has a lot of moon symbolism behind him.
Another important distinction: Yang represents the outward expression of energy. It is doing, planning, creating, and moving forward. It is the effort you put into achieving a goal. Akutagawa is driven by a singular, active goal. His entire existence is a relentless march to force Dazai to acknowledge his strength. Every battle he fights is a calculated move to prove his worth through physical dominance. He does not pause to weigh moral consequences, question his worth, or overthink a situation. If a target is in front of him, he strikes immediately with lethal intent, even if it would be wiser to keep said target alive.
Yin, meanwhile, is resting, waiting, reflecting, and being still. It is not "laziness," but rather the essential recovery period that allows for future action. Atsushi struggles with a lack of agency; he always hesitates and double-thinks his actions due to the trauma of the orphanage. Before he can act without hesitation, his arc is about working through and recovering from that trauma.
What I’m trying to say is that Atsushi has as much in common with Yin as he does with Yang, and vice versa. The second part of this post will cover my "smoking gun," so to speak, when it comes to this symbolism—the thing that made me realize I was ultimately right.
