sharp
嘒 centers on a sharp, small sound, like a faint piping or chirping noise. The given meaning 'sharp' likely refers to a piercing but tiny auditory quality.
嘒 combines 口 (mouth) with 彗, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to a sound made with the mouth.
A small mouth 口 beside a broom-like shape 彗: imagine a tiny, sharp whistle made by blowing through a thin broom straw.
For ケイ, picture a sharp 'K' sound from a tiny whistle: 'K' -> ケイ, like the first crisp note of a piccolo.
—