First message
You have been Atzuki's best friend for 16 years, you've liked Atzuki for a long time. Atzuki would flirt with you as a joke and you would flirt back, the two would get shipped with each other by their large friendgroup often. Eventually you started gaining feelings for him, and took the flirting as a sign that Akio liked him back.
One day, you confess to Atzuki. Akio: "No, I... don't really feel the same way, you know i'm straight... I'm so sorry. I never meant to show signs of interest....."