When I read the manga though, it was different with kunikida as kunikida actually believes the hanging rope idea as it took place when they first meet atsushi
Well, I guess this take was just a comical scene about how naive Kunikida can be, making Atsushi wonder how this strange duo could be detectives.
I could end the answer here, but I'll go into the theme because there's much more to say. Long post warning!
⚠️Spoilers from the manga and the novels (especially Dazai's entrance exam, 55 minutes and Untold Origins).
If you analyse his character, you'll see that he's not really naive. He's incredibly sharp and quickly understands the situation (oh well, ofc not at the same level as Ranpo or Dazai). He doesn't let himself be fooled by his enemies, and when that happened it was because the enemy took his ideals and tested them (see Azure Messenger and Fyodor).
You know, I hate that Dazai is manipulative but it’s also part of his personality. I’ve seen some analyses that say Dazai is also testing Kunikida's ideals and I totally agree. However, unlike Fyodor, Dazai never did this to break Kuni (despite his questionable actions).
In my opinion, this happens because Kuni trusts Dazai. He lets it happen since he knows that Dazai doesn't intend to hurt him or the people he cares about.
At least to me, it seems like knkdz duo is incredibly unbalanced, with Kunikida being fooled by Dazai every time while Dazai hides his true feelings.
Ofc when we talk about the ANIME (which did a HORRIBLE WORK of showing their relationship)
Unfortunately, the manga doesn't delve into their relationship as much as I remember. I don't blame it though, because the manga focuses on Atsushi's relationships and world-building.
Thankfully, we have the novels to fill in the gaps.
In general Kunikida is aware of Dazai's character. He knows that Dazai isn't exactly a good guy (even before learning about Dazai's past).
In Dazai's entrance exam, when they first met, he sensed something strange about the newcomer.
Even Fukuzawa is aware of how suspicious and dangerous Dazai is. Due to Ango's ability, they don't know Dazai's past and are suspicious of him.
For me, Dazai's entrance exam represents Kunikida understanding the consequences of his ideals and Dazai trying to start a new life (if we didn't know Dazai due to manga/anime I would say that he was the villain) plus they start to understand each other.
We see their relationship grow, and after resolving all doubts, Kunikida says this line:
He trusts Dazai and Dazai knows it.
Kunikida seems to hate Dazai. He actually admires him. He trusts him. Despite knowing he can't solve the problem alone, he asks for help. Look what happened in 55 minutes. Kunikida is Atsushi's senior and saw him in trouble. He couldn't help, so what did he do?
Not to mention the chapter where Q's ability cursed Kunikida.
Although we haven't seen many many moments where Dazai showed his true feelings, we have this moment. Dazai is also aware of Kunikida's qualities and undoubtedly trusts him as well.
Maybe I changed the path of the answer too much, but there's one thing to say. Kunikida cares about Dazai. See, in 55 minutes, Kunikida's reaction to the news that Dazai was about to die.
He got very angry. It doesn't mean that he wanted to kill Dazai literally. It was an outburst of anger.
To finish, everyone (at least in the ADA) acknowledges the importance of this duo, and it always makes me happy.
(A Day at the Detective Agency excerpt/ included in Untold Origins novel)