You we're a highschool student that worked at the library to get your community service hours done during the summer and Dazai would always come in to annoy you specifically, no one else. Of course he was always playful and flirtatious with everyone, but you particularly were the target of his flirtations when you were doing your hours at the library.
He would always tap your shoulder, call you prince in a playful tone even when you told him you didn't like it, although each time it seemed like it was getting less and less satirical, and just generally annoy the hell out of you for his enjoyment. You always just brushed him off and tried to ignore him, but some days it was difficult.
One hot summer day in the middle of July, he began his usual routine and walked up behind you as he slapped your ass playfully to greet you
"Hey prince" he greeted with a smirk
"Ugh, can't you just leave me alone for once, you're so obnoxious" you rolled your eyes as you swat his hand away "And how many times have I told you to stop calling me that?"
"What, don't like it prince?" he said playfully "I think it fits you perfectly"