Atsushi has heard that a person’s bento box says a lot about the person preparing it, and their relationship with the person it’s prepared for.
He isn’t sure that’s true. When he joined the agency, he prepared his own lunch. The box he used was a plain one (the cheapest he found that didn’t instantly fall apart) and it was almost never completely full.
…Maybe it said that he didn’t have a lot of money? Or maybe it said that he wasn’t used to being able to eat his fill, and always kept something in reserve in case he was about to be thrown out and left to find food on his own. Maybe there’s something to what he heard after all.
In that case, what about Dazai? Dazai’s brightly-colored bento box is always neatly packed, with not even a grain of rice out of place. There’s always something delicious for dessert. Considering how poorly Dazai takes care of himself, Atsushi’s surprised that he even remembers to pack his lunch every day, much less pack it so neatly and so full. He wouldn’t be surprised if Dazai only brought alcohol for lunch.
The subject of Dazai’s bento box remained a mystery at the back of Atsushi’s mind, up until the day when Dazai slings an arm over Atsushi’s shoulder during lunch and says, sing-song, “Atsushi~ Let me have some of your lunch?”
Atsushi holds his box closer to his chest. “You have your own lunch.”
“Actually, I forgot it at home,” Dazai replies. “And I can’t leave in the middle of work… You wouldn’t let me go hungry, would you?”
Just Dazai claiming to be unwilling to leave during work is enough to make Atsushi suspicious. “I’m sorry to hear that. Be more careful to remember tomorrow.”
Dazai pouts at him, before turning to seek someone else he can convince to part with their food. “Kunikida~~”
Kunikida doesn’t even look up from his computer. He’s taking bites of his food and chewing each exactly seven times while he continues to work. “No.”
Ranpo, with a mouth full of candy, tells him: “No way.”
“You can buy a water bottle,” Atsushi says, and returns to having lunch.
Dazai’s pout intensifies for a few more minutes before the front door opens, and a redheaded man sticks his head inside. “Dazai, you forgot your lunch.”
All heads in the agency - minus Kunikida’s, since he’s far too busy working - turn towards the door.
“Odasaku~!” Dazai springs to his feet. “You’ll spoil me!”
Odasaku is smiling very faintly as he holds out Dazai’s usual bento box. “I put extra crab in to make up for it being late.”
Dazai’s smile as he accepts the box seems strange to Atsushi for some reason he can’t quite pin down for a few moments, before he realizes: ah. This is Dazai genuinely happy. That’s a bento box packed with care by someone who makes Dazai smile that way.
One mystery solved, a million more discovered.