light
耿 centers on light, specifically a clear, bright, or luminous quality. It can describe literal light or a metaphorical brightness such as clarity of mind or upright character.
耿 combines 耳 (ear) and 火 (fire). The exact historical origin is uncertain, but it likely expresses brightness or clarity, possibly from the idea of firelight illuminating or a metaphorical extension of clear hearing.
Imagine an ear 耳 catching the glow of a fire 火, making everything bright and clear. The fire's light illuminates the ear, symbolizing clarity and brightness.
For コウ, picture a bright light shining on a 公 (こう, public) square: 公 -> コウ, and the fire 火 makes the whole area bright.