It wasn't as if you didn't know about soulmates.
You knew about them, everyone knew about them and despite the fact that the orphanage tried so hard to drill in you brain that you won't ever ever meet your soulmate and you should just give up, there's always a small flicker of hope inside of you that you'll meet your soulmate soon. (Truthfully, during the three days you had to survive out of the orphanage, you were scared. If you really had a soulmate, wouldn't dying now mean not meeting each other? Would you not deprive of them in color by your death? Even in death, you still bother people.)
You were a little envious of the people in the agency if you had to say truthfully. During your first encounter with Dazai, you'd known he'd found his soulmate after remarking on the color of your eyes. You later asked Kunikida about this later and he shrugged, telling you he can't see all colors despite finding Dazai. He does tell you that part of your eye is a vibrant and deep purple. (Someone then informed you the both of them didn't have one but two soulmate and that's why Kunikida could not see colors with a reddish hue to them despite Dazai seeing all colors and often times you would wonder why Dazai would not introduce Kunikida to his other soulmate.)
You often fantasized about meeting your soulmate soon, and finally seeing color for the first time. You'd think about the deepness of red and the lightness of blue before remembering and trying to banish all thoughts of your soulmate. What if the orphanage had been right? What if-
What if you don't have one?
You shake your head, you know that can't happen. The gods above can't possibly be so cruel as to deprive you of something that 'completes' all humans. To deprive you of seeing color. (Though, there's something inside of you that can't shake off the feeling that perhaps the gods would be this despicable. Torturing you with the knowledge that someone's out there for you when in reality, there's really no one.)
The first time you saw him, it was only a picture. It was only a picture and you knew he was formidable, he was scary, he was- he was not someone to be reckoned with. You were a little bit scared of him, seeing how Kunikida (someone responsible and to be trusted) warned you about him. You could not help but stare into his eyes, black as the void and as deep as it too. You could not see color but you think right there and then that his eyes might be pretty in real life.
The second time you saw him- the second time you saw him and then knew was in battle. He has perhaps killed two of your friends that was just trying to help you know the ropes of this job, and you were right. He does have pretty eyes in real life.
You falter and almost vomit right then and there as color suddenly swarm your vision and overloads your sense of perception. It's- it's too much and you can finally see the color that you've so long wanted. Of course the gods would not be so despicable to deprive you of color, but they would be so despicable to give you a soulmate that hates your guts. (The gods- their only entertainment would be the trifles of men.)
Did you-
Did he falter when he realized? You do not know, rather, you did not see since you were too busy trying your best to not vomit and empty your stomach at such a crucial time. One hand at your stomach and the other covering your mouth, and you finally look at him in person, in real life.
He looks as if he comes from the night, he looks as if he is Death himself. You cannot help but fall a little. (alas, all soulmates; weather they hate each other or not- they're doomed to fall.)
Later, when Dazai finally rescued the three of you and when you're told that your coworkers-friends were not dead but only injured, you can't help but ask Kunikida for the picture he has on him. The older man just rolls his eyes, muttering something about you being suicidal as fuck, and you weakly grin at him as he gives you his picture.
Once you are in the safety of your home, (it's been so long since you've thought of something as home) you can't help but speak of his name out loud. Testing the taste and how it feels on your tongue. It feels wrong. It feels disgusting. It feels right.
The gods truly hate you, but even if he hates you, even if Akutagawa Ryuunosuke hates you with every part of him, even if the hate goes deeper to the bones, the picture will still remain with you, as if a charm.