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Season 1, Episode 9(/10) Chuuya tells Dazai that he opened a bottle of wine in celebration. Personally, I believe he did it in grief that his partner left him
After Dazai left, Chuuya could never use the strongest level of his ability, Corruption. It leaves one to wonder if Chuuya ever thought of using Corruption to see if Dazai will make it in time. If he didn't make it, then he wouldn't have to suffer anymore.
Chuuya probably has his own issues on the topic of being human or not. It makes me wonder if he sometimes just feels like a vessel for Arahabaki rather than his own person. Whenever Dazai is using his ability on him he feels the most human and normal because he can't feel the God of Destruction.
Every year on the day Dazai left, Chuuya would drink himself unconscious. If he went out to a bar to do this, Dazai would always come and take him home. He would be gone before Chuuya would wake. He never questioned the water and pain relievers on his nightstand or how he got home. He always assumed it was someone from the Black Lizard or another Executive.