Ranpo is presented as blunt and childish, and ridiculously so. He goes around telling everyone about how he’s the greatest detective and is better than them, yet Atsushi has to assist him on a case simply because he doesn’t know how the train system works. 

The only thing I’ll say about the murder case itself is that it’s BSD’s earliest example of governmental and police corruption, a theme that will appear multiple times throughout the manga, especially in the Hunting Dogs arc later on. Corruption of this sort is in of itself an absurdity of life, and a very common one — and an organization like the Armed Detective Agency is a force that pushes back against this absurdity. In this chapter, Ranpo serves as the rebellious force doing so. 

Detective Minoura is very dismissive of Ranpo and the agency, claiming multiple times that Ranpo basically an immature hack who couldn’t possibly solve the case. He’s clearly very frustrated by Ranpo’s presence and is very quick to assume the worst in him just because of his childish demeanor. This is played up for the sake of dramatic irony, but I also think it’s representative of Minoura’s (and the institution of police as a whole’s) tendency to overlook those who don’t hold power over them. 

Dazai joins the gang once he’s pulled from the river, claiming to have been “enjoying the current” and seeking a woman to enact a double suicide with. I do believe this is a front, though, and he just needed an excuse to be a menace and to be involved with the case so he could point out to Atsushi later that Ranpo does not, in fact, have a special ability at all (although this is just a theory of mine). 

The fact that Ranpo doesn’t have an ability but pretends he does is extremely important to his character. While I have not read the Untold Origins light novel, I did watch the adaptation in season 4 of the anime, and although I understand the anime adaptations of the light novels leave a lot out, it did emphasize Ranpo’s struggle to connect with others. He referred to himself as a “monster,” and was unable to relate to the people around him after his parents died. I bring this up because it's the reason he pretends he has an ability, it gives him an excuse for being different from others. 

What absurdists often do in their storytelling is exaggerate something absurd about our reality in order to bring the reader’s attention to said thing, and the presence of abilities in BSD allows for that to happen. Those with abilities are often outcasts of society, and abilities are also the catalyst for conflict in the BSD world. Akutagawa in chapter 3 was a great example of this — because random violence does exist in our real world, but it was exaggerated and made even more horrific because of Rashoumon’s destructive power. 

Ranpo’s non-ability ability is an absurd paradox, then.

Up until the end of the chapter, we’re made to believe that Ranpo is an ability user. While the idea that someone can simply solve a mystery in mere minutes because they have a magical superpower is already absurd, you know what’s even more absurd than that? The person in question just pretending to have a superpower and solving the case solely by use of their intuition.

Ranpo’s masquerade of having an ability is his way of combating an absurd reality because if he’s not an ability user, he’s a freak. But because he gets to pretend to be an ability user, he not only has an excuse for his crazy intellect, but everyone has to at least tolerate his odd personality. It makes him feel less othered. 

Is it convoluted and a bit insane? Perhaps. But that’s kind of the point of embracing the absurd. The ways in which characters often revolt against the absurdity of life is by being somehow even more absurd, by doing something even crazier. 

So why does Dazai make a point of allowing Atsushi to believe Ranpo is an ability-user for the whole case, only to reveal afterward that he is not? He could have told Atsushi from the get-go, or he could’ve not told him at all, so why do what he did?

Apart from the fact that it just makes for more suspenseful storytelling (remember, this is only the sixth chapter, Asagiri is trying to keep the audience engaged right now as he builds his character and world), I think that Dazai is teaching Atsushi to not take everything at face value. Ultimately, he’s training Atsushi to be a detective, and that means that he should be questioning everything, considering every possibility. 

This idea lends itself to absurdism because if you don’t question the supposed truths of reality, you’re essentially giving in and allowing the absurdity to control you rather than pushing back against it. To Dazai, this is a way to train Atsushi to look at the world differently — he presents him with an apparent truth (Ranpo is an ability user) and then forces him to bring that into question, to expose the absurdity (that Ranpo is just ridiculously smart).