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Atsushi didn't know much about love, never having much of it for the majority of his life.
But he had started learning about it now, since joining the Detective Agency.
He was learning how Kunikida's strict schedules were a form of love, how he never let anyone skip a meal or sleep less than eight hours at night. He was learning how Kenji's smiles were all genuinely lovely, never hiding anything behind them. He was learning how Yosano's weird medical experiments were out of love too, how she studied all of their bodies so thoroughly so she could bring them back time and time again, their breath pattern itched in the back of her mind.
Atsushi was even beginning to understand Dazai's love too, more or less, bit by bit.
He didn't love working, but he respected Kunikida so he came to work sometimes, he didn't love kids, but he always indulged Kyouka, and he proposed double suicides to women all the time but, apparently, he was married to a man.
Not any man, he was married to Nakahara Chuuya, Port Mafia Executive.
Atsushi might not know much about love but how could Dazai be married to someone from an enemy organisation he had escaped, someone who, to the naked eye, he seemed to hate?
How did that work?
Since joining the Detective Agency, Atsuhi had learned a lot about love.
There were all kinds of love, the one Yosano and Dazai had (born out of a mutual understanding of their time under Mori's command and how that affected people) was different from the one shared between Ranpo and Fukuzawa (born out of two people who saved each other) but it didn't make it any less important. Then there was the friendship between Yosano and Rampo, product of two chaotic souls meeting, or Kunikida and Dazai, partners in crime and justice. Atsushi liked to think he was intertwined in their web too, new threads entangling them more and more each day that passed, marking them with tenderness and affection.
However, despite all the different types of love he had discovered since joining the Agency, they had all one thing in common, they were all without malice.
Yosano's and Rampo's pranks could be a bit annoying, and Kunikida could be too strict, Fukuzawa too distant, but there wasn't malice in any of their actions.
Because love, as he had come to learn, was supposed to be nice and well-meaning.
If not, it wasn't really love.
And, nice and well-meaning weren't the first words that came to mind when one thought of Nakahara Chuuya and Dazai Osamu's relationship.
Dazai mocked him at every turn, almost viciously, he dismissed him, he ignored him and, ultimately, he had left him for four years. Nakahara did his worst too, he would kick him, shout back just as viciously and insult him every three words.
There was nothing tender or adoring in the way they treated each other, there was no love there.
At least, that's what Atsushi thought when he first came to know about their marriage.
That was until one winter morning, several weeks after the start of their truce with the Port Mafia.
Atsushi was running late for the first time ever, panicking about what Kunikida would say to him and wondering why Kyouka hadn't woken him up, when he saw Dazai and Nakahara standing in front of the Agency, discussing something. Atsushi must have been terribly late because he always arrived before Dazai, everyone did, so he started running faster, the conversation between the two men reaching his ears despite the distance between them still being great thanks to his tiger senses.
'...like a fucking toddler, I swear to god, Osamu, don't you know how to fucking dress for winter like a normal person?'
'If anything is like a toddler, it’s your height, chibi.' Dazai stuck out his tongue. 'And I don't listen to fashion advice from hat-racks.'
Nakahara kicked him in the shin, making Dazai whine and double over, which Nakahara used to his advantage to loop his blood red scarf around the tall man's neck.
'Shut the hell up, mackerel.' Nakahara grumbled as he half-strangled Dazai with the scarf. 'If you catch a fucking cold, I'm kicking you out. I won't let you contaminate my apartment with your shitty germs.'
'Our apartment, chibi.' Dazai corrected, looking down at Nakahara with something akin to fondness. 'And did our vows mean nothing to you? In sickness and in health! How could you abandon me now, in my hour of need!'
Nakahara finally looked up at him, and Atsushi could clearly see how hard he was frowning. 'Just watch me. I'm gonna find that marriage certificate and burn it to ashes, so I can finally get rid of your annoying ass.'
'Good luck finding it and all the copies I made.' Dazai tried to smooth down Nakahara's frown, unsuccessfully. 'You can't return me without the receipt, love.'
Then, as if the pet name wasn't enough to make Atsushi trip and fall head first into the pavement, Dazai leaned over and kissed his husband.
And, that, more than anything, convinced Atsushi that there was another type of love, a love that may appear rough and mean at first but was just as nice and warm as the others.
Because, when they kissed, eyes closed and sharp words hidden, one could see the love between the two men as clear as day. Dazai's hands on Nakahara's cheeks, Nakahara's hands gripping Dazai's scarf to bring him closer, and both of their faces devoid of any anger or mocking, smiling sweetly into the kiss as if it was their first one.
People that kissed like that, with their whole body into it, could only be maddening in love, in Atsushi's very limited opinion.
Atsushi reached them as they separated, blinking lazily and cheeks baby pink, and Dazai sneaked another two quick pecks before finally turning to Atsushi, both his hands still firmly planted on his husband's cheeks.
'Oh my, oh my, Atsushi-kun running late! Kunikida is going to be so disappointed in you.' He chastised him, his broad smile too genuinely proud for him to really mean it.
'Then you're also late, idiot.' Nakahara took a step back, slapping Dazai's hands off him. 'Stop lazing around and go already!'
'But chibi was the one distracting me with his chibiness and making me late! Where's your accountability?'
Nakahara's cheeks turned red with fury then. 'Me? You waste of bandages, I'm gonna show you...!'
Atsushi then entered the warehouse, leaving them be.
There was still so much Atsushi needed to learn about love but he was making progress bit by bit.