Soukoku has ruined it for me...
Any other pairing for Chuuya and Dazai in whatever context you wanna take it - platonic, romantic, found family, soulmates, enemies, frenemies etc etc - one can't be without the other or in the same capacity what they have with each other...
Like, I can never imagine Dazai getting frustrated with anyone and downright saying to their face - 'You're no ray of sunshine either' - other than Chuuya
or Chuuya ever saying - 'I used corruption because i trusted you' to anyone other than Dazai (tbf, he literally can't say even if he wants to, unless it's another nullifier)
or Dazai spending day and night, thinking up ways to annoy anyone but Chuuya
or Chuuya doing a rich girl impression or equivalent just because someone said he will do so
or Dazai talking about how anyone's ability is too overpowered and its final form, sound equal parts smug, fond and worried
or Chuuya looking at any agency fight and thinking, if it was Dazai there won't have been a fight to begin with
or Dazai planting a bomb under any of his friends/colleague's car, fully certain it won't harm them
or Chuuya shooting his subordinates out of their discussed plan, just to shut them up because they were getting cringe
Or you know - the in general - devising a whole plan, heading into enemy's side, getting stabbed with poison and trusting the other will save you, without any sort of communication what so ever.
And the said other person, doing exactly as was expected of them and activating the uncontrollable part of their ability, without any confirmation of the only person known to deactivate is even alive.
One shotting the other in head, the other letting it happen.
One trusting the other with their humanity, and other making sure to keep it intact.
One getting kidnapped and leaving a clue that only the other could connect.
I mean, you could argue one or two of these things can happen with either half of Soukoku and someone else, but all of these?
They may have n number of different kinds of relations with all the other's from bsd verse, but what they have with each other is unique and it's only them who will have that kind of thing...
I'm not saying they are a ideal - friends, enemies, couple.... Partners
What I'm saying is, what ever they are to each other - is the best they will get... it's one of a kind - only piece available in the whole world