sincere, honest, depressed
愊 centers on a heart/mind state of sincerity and honesty, but can also describe a heavy, depressed feeling. The meanings connect through the idea of an unguarded, genuine inner state, whether open and true or weighed down.
愊 combines the heart radical 忄 with 畐, which likely contributes the sound. The exact historical development of 畐 is uncertain, but the character as a whole expresses inner states of sincerity or depression.
The heart 忄 beside 畐 (a container full to the brim) suggests a heart so full it cannot hide its true feelings—whether that fullness is honest openness or heavy sadness.
For ヒョク, imagine a person so sincere they 'hyuck' (a soft, earnest laugh) when they speak from the heart: hyuck -> ヒョク.