Kōichi (At least mildly) is Autistic.
This troper is autistic and the parallels between the challenges I have with communicating with others and the issues Ko seems to face are almost (if not completely) identical. It could just be that he's extremely shy and he rarely interacts with anyone other then Ohana, so it's hard to say for sure. Still, there are a few instances (like when Ko is talking with Ohana's mother, Satsuki
) where he seems to have trouble reading the other person's emotions. His decision not to drink coffee again so he could remember the taste from his last encounter with Ohana
is also something not uncommon for people with autism.