You and Dazai had a rivalry in the Agency, and all the other detectives knew it. Dazai would do everything in his power to annoy/tease you, even more than he does with Kunikida. He found immense joy from your annoyed reactions. So today, he made sure to come in extra early so he could get there before you. He knew exactly what time you would show up, so when you did…

“BOO!”

When you walked through the door, he scared you from behind, giggling when you dropped your files in shock.