Ah, tough question...

It's a very silly/inconsequential one, but I headcanon that Adachi really likes sweet foods. I also headcanon that he tries to hide that around others. (There's this scene where he drinks black coffee in 'Fidelity Decay' that I think about a lot, haha.) I try to work that headcanon into most of the fic I write with him. On a similar note, I'm one of those people who thinks that Akechi dislikes sweet foods.

I also think that he's pretty good with (young) kids, though he doesn't like interacting with them. (Nanako being an exception, of course.) He might be a jerk to them unintentionally, but he'll try to make them feel better if that happens, and he's good at understanding their moods and needs...even if he doesn't want to be the one responsible for managing those things.

I've been dumping a lot of my Adachi headcanons into my main WIP because that one is literally just me writing everything I want...but I also consider that Adachi very different from canon, so. Hm...

For Yu, I like to think that as composed and together he seems around other people, he's actually a bit of a mess. (I guess that's not an uncommon headcanon...) Stuff like him being clumsy when he's alone, and his eating habits being absolutely unholy when he doesn't need to cook for someone else (amazing cooks who will sniff questionable food and go 'good enough' are a soft spot for me), to the far extreme of being a concentrated ball of anxiety and stress that just masks it really well... How much of a mess I make him depends on the fic. I do genuinely love actually hyper-competent Yu Narukami as well, but I find a messier boy more fun to write.

I also think Adachi has an average to large-sized dick, cause I think he deserves it <3 I always headcanon Yu having a bigger dick though. (I'm a simple man.)