happy
悆 centers on a state of happiness or contentment. The single given meaning 'happy' captures its core idea.
悆 combines 余 (likely phonetic) with 心 (heart/mind) to express a happy or contented state of mind. The exact historical development is uncertain.
The top 余 can suggest surplus or ease, and the bottom 心 is the heart. Imagine a heart with more than enough—overflowing with happiness.
For ヨ, picture a happy person saying 'Yo!' with a cheerful wave: Yo -> ヨ, and their heart is full of joy.