prepared for, inclusive
賅 centers on completeness and readiness: being fully prepared for something, or covering everything inclusively without gaps.
賅 combines 貝 (shell, money) with 亥, which likely contributes the sound. The exact historical development of the meaning 'prepared for, inclusive' is uncertain.
Shells (貝) were used as money, and 亥 looks like a container. Imagine a merchant with a full money pouch, completely prepared and including everything needed for trade.
For カイ, picture a fully prepared kite (カイト) that includes everything needed to fly: kite -> カイ, and the kite is ready and complete.