Pumpkin

Dazai uses what he knows about people to get the information he wants. He won't see the full picture immediately, but he has several times in the show used the tiniest clues given to him by people to deduce what it is. Much like Mori, he is able to put people exactly where he needs them to be. He also as a sixth sense on how to best support his partners because of this. He's smart on his own but I firmly believe that most of his manipulate genius lies in what he observes in those around him. For this reason, he is susceptible to being tricked and harmed because people cannot always be predicted or trusted. On top of that, he does not always want to believe in what he's seeing.

Ranpo lacks an understanding in people and seems a bit detached from people in general. When he's younger, we find out that he truly does not understand why people act the way that they do. His intelligence is completely independent of those around him. He just sees things. It's all seemingly mapped out for him at just a glance. Even Dazai admits that Ranpo is on a different level entirely. Despite this, he lacks common sense. I'm personally not unconvinced it because he enjoys the affection of having people tend to him since he seemed to have a rough life before Fukuzawa got him. Ranpo is the stereotypical anime genius. He seems to fail the most when people have gone out of their way to trick him, or "alter up the big picture" so to speak.

This does not mean they don't have chemistry. They absolutely do and they are a fan favorite for good reason. I don't think their relationship would ever surpass a complicated situationship. When it comes down to it, there's too much distance between them. It's obvious that they both care about each other. They do well to make sure they would never die and know each other well enough to be soukoku regardless of how often they have seen each other.

But when there's no near death experiences happening, their dynamic is not intimate enough. Chuuya had the unique position of being a teenager who could treat Dazai like the annoying teenager he is. I don't doubt that Chuuya has a deep understanding of Dazai that rivals most people. However, he was also a teenager going through things and wouldn't have the perspective of life that would let him see Dazai the way Oda did.

Dazai would never let Chuuya get so close, but he definitely values Chuuya. He enjoys his company, finds his humanity fascinating, and gets a kick out of annoying him. Chuuya doesn't care to let Dazai get too close either, but also holds his opinion in high regard, calls him while drunk, can scream his name even while in corruption, and even humors their embarassing inside jokes. They don't have to be close or completely understand each other to value each other, but they will never be intimate enough on a casual basis to be relationship worthy.

For starters, Dazai is not a heartless person. While he claims that evil and good don't sway him, I don't think that's the entire truth. People do sway him, regardless on if they are good or not. Dazai genuinely loves or at least cares for them, but does not express it normally.

Chuuya wanted somewhere to belong and to believe he was human. Though he never asks these things of Dazai, it doesn't stop Dazai from delivering them. He all but drags Chuuya into the mafia and places him in a partnership so devastating that they're still going on missions as that duo while being enemies. He is shameless in his compliments to Chuuya. Though he irritates Chuuya, it's all done with a precision that shows that Dazai pays attention and cares about him. It won't save Chuuya from pain, but Dazai would never allow Chuuya to die if he could stop it.

Kunikda has extremely frigid ideals. Dazai challenges this constantly because he is aware that the world is not so idealistic. Even in the smallest ways, such as disrupting his schedule, he is challenging Kunikida to grow. Their partnership is to an extent sweeter than Chuuya, but does involve some soft occassional interference disguised as being a nuisance.

It even goes to Akutagawa. While the way he treats Aku is not okay, he does not hate him. I'd argue that Dazai actually really likes Aku. He speaks highly of his power to Oda, spares his life despite his insubordination (as they are in the mafia, Dazai could have easily killed him for disrupting an important mission), and views him as worthy enough to be part of the next Soukoku. However, his way of driving Aku into his full potential is through forcing Aku to view him as a superior who's acknowledgement is worth more than his life. Dazai fills the gaps left behind with violence. He is absolutely capable of being tender with Aku, he just thinks that it would devalue his authority to Aku and so maintains a cold demeanor even to this day. Even as Aku has come to hate him, he still idolizes Dazai. It won't matter to Dazai because he delivered in his promise to give Aku a purpose. He has successfully made Aku stronger by introducing Atsushi into his life.

In any relationship Dazai has with people, he is achieving something whether they have asked him to or not. He doesn't seem to care about their perception all that much. If he is seen as lazy, abusive, terrifying, it's all the same to him. Still, Atsushi surprises him because his assumptions of him are not negative. By all accounts, he is as manipulative to Atsushi as he is with any member of the agency. He is as roundabout with him and even has a moment of cruelty that mirrors his time with Aku. He has heard about who Dazai was and has seen who he is at the moment. With all of that information, he seems to be the only one who has vocalized to Dazai that he views him as a good person.