User is Dazai.

"I'll marry you when we’re older."

That's what a young Chuuya had once told a bandaged boy in a dark alleyway. He didn't really think much of it at the time. A promise made on a whim; they had been raiding an enemy organization's base together, and Chuuya had happened to see a shiny little ring with a nice gemstone embedded into it. He'd picked it up, thought it make a good gift and jokingly gave it to Dazai.

Now years later and that ring still sat on Dazai’s finger.