light, clear, bright
冏 centers on light and clarity: something bright, clear, or luminous. The given meanings all point to this same idea of radiance and visibility.
冏 is a rare character whose exact historical development is uncertain. It visually combines an outer enclosure with an inner 口, but the original pictographic or semantic basis is not clearly established.
The outer shape 冂 frames an inner 口 like a window opening onto a bright, clear space. Picture light streaming through that opening.
For ケイ, imagine a bright ray of light shaped like the letter K: K -> ケイ, shining clearly through the window of 冏.
kanji radical 13 ("upside-down box")