star
呇 centers on a star, likely a single bright point of light in the sky.
The exact historical development of 呇 is uncertain. The modern form visually combines 水 (water) and 口 (mouth), but this may not reflect the original character structure.
Picture a star reflected in a pool of water (水) that you can almost drink from a small opening (口). The star's light shines through the water's surface.
For ケイ, imagine a bright star named Kay: Kay -> ケイ, shining above the water.
—