Bungou Stray Dogs
AU where Soukoku were lab rats TOGETHER and experimented on TOGETHER so Chuuya's not the lone one.
So they meet during the labs and experiments and they're curious who the fuck this other kid is because for as long as they've known they've been the only one.
And somehow they're practically MADE for each other????
I mean Dazai counteracts Chuuya's ability which is super dangerous and Chuuya's strength easily overpowers Dazai's so he could theoretically take him out in a second.
And somehow during the process the experiments turn them immortal? Because I mean why not.
It would also kinda be ironic because Dazai would be an immortal trying to die.
But also that could be the whole point of Dazai's suicidal tendencies because he can't die so he wants to die.
But because of their shared experiences they're really dependent on each other and they stick together.
Even when they're with the Sheep group and Dazai obviously knows they're being used.
In a way Chuuya knew too but was being purposely ignorant about it.
So when someone comes to recruit them to the Port Mafia they're both skeptical and Chuuya outright refuses but Dazai considers it.
And in the end Dazai decides they should and he convinces Chuuya to abandon the Sheep together.
When Dazai meets Oda Chuuya is right there with him.
Thus consequently, when Dazai defects from the Port Mafia years later, Chuuya is still with him.
That's as far as I can think of but I feel too unmotivated to pick this up.