Imagine Dazai getting temporary amnesia and somehow only forgetting Chuuya. He meets Chuuya on a walk to the store and Chuuya just gets ready for their usual banter and Dazai's just like "....who are you?" and Chuuya thinks he's just joking at first but Dazai doesn't seem to recognize him so he grabs his bolo tie and yanks him down to his level, examining his eyes for any source of recognition (or brain damage) and Dazai is just like "tf?"

(As for what caused it, probably too much alcohol and a vitamin B12 deficiency idk I just like the concept XD)