This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form combines 金 (metal) with 惠, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: metal 金 beside 惠. Imagine an ancient metal tool whose purpose has been forgotten.
For ケイ, picture a mysterious metal object shaped like the letter K: K -> ケイ, and the metal 金 component reinforces the scene.