There's something deferential and polite in how Ranpo won't unravel Dazai. I don't think it's that he couldn't. I think that Dazai exudes the panicked, defensive air of a prey animal, and Ranpo isn't interested in affirming Dazai's expectation that everyone he meets might decide to drag him from where he hides and eat him. People aren't monsters, Ranpo knows, but he also remembers feeling otherwise.
And so Ranpo calls him an enigma, sits beside him rather than opposite of him, allows Dazai proximity enough to touch Ranpo and confirm for himself what Ranpo doesn't want to admit, each gesture a steadfast assertion that Dazai doesn't exist in contrast to others, and the presence of others needn't be a confrontation or threat. Ranpo doesn't need to violate Dazai by prying into his totality to trust him, nor is Ranpo going to coddle him by pretending he exists in absolutes that could ever even be completely deduced. Ranpo doesn't ask or try to induce Dazai to spill his guts or unravel his raw nerves to earn the Agency's trust or satisfy Ranpo's own interest. Ranpo only needs to know Dazai enough to recognize and acknowledge Dazai's needs and alignment with the Agency's purpose—his distrust of hypocrisy, his desire for privacy, and his yearning to find purpose in doing good won't solve any mysteries, but they're all Ranpo needs to know.
(Because Dazai isn't a mystery. He's a person. Ranpo isn't interested in him for anything other than that.)
It helps that, as alluded to above, Ranpo doesn't ask anything of Dazai he isn't submitting himself to, too. Scrutiny is not something Ranpo often has to think about, but he has every reason to be defensive around Dazai, who could expose him to his own sources of discomfort either intellectually or with No Longer Human. Instead, Ranpo models for Dazai the placidity, trust, and deference he expects from Dazai, and perhaps that's why Dazai submits to molding his thinking around Ranpo's, makes paper airplanes parallel to Ranpo's snacking, and chooses not to read artifice in Ranpo's gestures.
Ranpo will not seek to demystify Dazai, and Dazai won't seek to dissect Ranpo. They don't need to glance at the other's shadow to share the same waning sunlight. More to Asagiri's point: they can't face each other so long as they're facing the same direction.
