This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
悈 combines 忄 (heart/mind) with 戒 (admonish/guard). The exact historical development and original meaning are uncertain.
The heart radical 忄 beside 戒 (admonish) suggests a heart that is warned or restrained. Picture a guarded heart, but remember the meaning is unclear.
For カイ, imagine a cautious mind (忄) giving a warning cry: 'kai!' — the sound cues カイ while the heart is admonished.