cheekbone
頯 refers specifically to the cheekbone, the bony ridge below the eye.
頯 is a rare character. It combines 頁 (head) with a left component that may relate to the cheekbone area, but the exact historical development is uncertain.
The right side 頁 means head or page. Picture a head with a prominent cheekbone highlighted on the left side of the character.
For カイ, imagine a person named Kai with very high cheekbones: Kai -> カイ, and you remember the cheekbone by picturing Kai's face.