to rely on, look up to
忋 centers on depending on someone or something higher: relying on support and looking up to a person or ideal.
忋 combines 忄 (heart/mind) with 己 (self/oneself), suggesting a mental state of relying on or looking up to another. The exact historical development is uncertain, but the component relationship is plausible.
The heart radical 忄 beside 己 (oneself) suggests a person's heart leaning on another: relying on someone and looking up to them.
For カイ, imagine a person looking up to a kite (カイト) flying high, relying on the wind to keep it aloft: kite -> カイ.