It is revealed by Etna in the Prinny World episode that Laharl's mother was a human Witch, who came to the netherworld to study. It was love at first sight between her and King Krichevskoy, leading to an instant resentment from Etna.
Laharl's mother tried to raise him with love, to be a kind boy. This backfired when she sacrificed herself, committing suicide in order to save Laharl's life. This resulted in him rejecting love.
The prinnies of the netherworld consist of the souls of those humans who have sinned. When they have worked off their sin, they ascend to the red moon, to begin another life. Laharl's mother leaves without a word. She is later seen hand in hand with King Krichevskoy (should the player achieve the game's good ending.