Soukoku.
Ah soukoku.
The fictional bungou stray dogs silly gay men.
Let's.. start from the beginning, shall we.
Back when these two were 15, Chuuya being what the SHEEP called their "KING" and dazai being the infamous demon prodigy, as always. The comment asagiri made on the way dazai was looking at chuuya the moment he saw corruption
"Even Dazai Osamu forgot how to breathe when he saw the storm that was Nakahara Chuuya."
Dazai absolutely ADORED chuuya as a kid, although their relationship was.. less than ideal. Chuuya was constantly mad at Dazai for being part of the port mafia and having to work with them seemed unacceptable to him. Except, Dazai, being on Mori's orders, was also secretly manipulating the SHEEP to betray chuuya to make him join the port mafia. The whole fiasco with Rimbaud and Arahabaki was one of the things that made that possible. They snuck out to arcades together, Dazai often sneaked into the SHEEP's hideout to meet chuuya.
Then when Chuuya first joined the port mafia, Dazai didn't see him alot. Before STORMBRINGER, Chuuya was with the Flags, a peer support group in the mafia, and after STORMBRINGER, Dazai got busy with Oda and Ango and Chuuya got assigned under Kyouyou. It was the time during STORMBRINGER that leaves a major effect here.
Dazai was the only person to COMPLETELY belove in chuuya's humanity, AFTER Adam. Hell, Dazai was prepared to risk the entirety of Yokohama and maybe even Japan just to prove that Chuuya is a human. Dazai's comments to Verlaine and Verlaine's replies all just proved how just.. determined this 16 yr old was to prove his "sworn rivals" Humanity.
"You make it sound as if you're certain he's human."
"There's no way I could hate a man-made character string this much."
"You messed with the wrong guy, Verlaine! I know exactly how to handle someone who can manipulate gravity. Day in, Day out, every waking and sleeping moment I've been thinking about how i can annoy chuuya."
Then in 18, after Odasaku's death, Chuuya was one of the only people to actually approach dazai. He even gave him that pendant of oda's birthstone which Dazai wears. When Dazai left the port mafia, Chuuya was obviously very angry. In his terms "I understood why he wanted to leave, but couldn't he had said something first?" Kafka even states that if Dazai told chuuya about his plans, Chuuya would've dropped EVERYTHING and went with him.
Then in the BEAST universe(Which is different from the normal series) Chuuya is highly protective of his boss, Dazai, even to the point that after Dazai jumped, Chuuya literally used corruption and burnt down most of Yokohama, even killing himself in the process.
Conclusion? JUST MAKE OUT ALREADY SMHH