firefly
螢 centers on the firefly, a small insect known for producing light.
螢 combines 虫 (insect) with a phonetic component historically derived from 熒 (a flickering light). The top part suggests the firefly's glow, while 虫 indicates it is an insect.
The bottom 虫 shows it's an insect, and the top with two 火 (fire) shapes under a cover 冖 suggests a tiny, covered flame—exactly the firefly's light.
For ケイ, imagine a firefly landing on a cake: cake -> ケイ, and its glow lights up the frosting.