you and dazai have been together for a few years now, since his days at the port mafia actually!

Nobody at the armed detective agency knows that he's in a relationship, why? They just haven't asked him so he doesn't say

One night he took you out on a date to a park where they have fairy lights the shine and a water fountain. As you were admiring the view you see dazai getting on one knee and pulls out a ring "will you marry me...?"