SKK AU where Dazai and Chuuya are high school rivals with a rather obvious crush for each other. Dazai fell first, and he fell /hard/, but Chuuya was the one who asked him out.

They've been mad love ever since the first date at the arcade.
At 16 they started dating officially,

at 20 they moved in together.

They are /happy/, which honestly is the least they could do after all the headaches they gave to their friends with their pining.

Until the day Dazai’s best friend Odasaku returns to Yokohama. Chuuya wouldn't say that Dazai /fell out of love/ with

him; it's just he seems to work best with Oda.
His smile is a little brighter and his laugh is a little richer, and the redhead has a hunch that, with Oda, Dazai won't fight as much.
So Chuuya would say he's not entirely surprised when Dazai leaves him. Sad, sure, and angry bc

because the breakup is messy. Surprised? Not in the slightest.

Two years later, though, Chuuya /is/ surprised to learn that apparently Dazai and Oda weren't quite so perfect, since they broke up.

And it's then that Chuuya is dared to seduce Dazai and break his heart as payback