brilliance
炅 centers on intense light: a brilliant, shining radiance. The single given meaning 'brilliance' captures this idea of strong, glowing brightness.
炅 combines 日 (sun) and 火 (fire), two sources of light, to express brilliance or intense radiance. The exact historical development is uncertain, but the component combination is straightforward.
The sun 日 sits above a fire 火, both blazing together. Imagine the combined light of the sun and a roaring fire creating an overwhelming brilliance.
For ケイ, picture a brilliant ray of light shaped like the letter K: K -> ケイ, and the K-shaped beam shines with intense brilliance.