open, peaceful
闓 centers on opening up or making something accessible, which extends naturally to a state of peacefulness or calm openness.
闓 combines 門 (gate) with 豈, which likely contributes the sound and may relate to opening or joy. The exact historical development is uncertain.
The gate 門 opens wide, and inside is 豈, which looks like a drum on a stand. Imagine opening the gate to a peaceful courtyard where a drum signals calm.
For カイ, picture a kite flying through the open gate into the peaceful sky: kite -> カイ, and the gate opens to let it soar.