Hotaru

Situation: When you are concentrating hard on something, or when you do something a little clumsy.
The truth behind the smirk: A heartwarming feeling of "It's cute, like watching a small animal" or "It's funny because they're trying so hard."
Character setting example: "The character is completely serious, but somehow airheaded, making them a beloved character who soothes those around them (especially younger people)."