As a Klingon much affected by her human mother's cynical upbringing of all things related to her father's people, special emissary K'Ehleyr was killed by Duras in 2367 as she nearly exposed the cover-up of his family's true treachery at Khitomer. Having once mated with Worf when he forswore a marriage two years earlier, she stunned him just before her murder by revealing Alexander, the son she'd bore by him in 2359 during their first encounter. A non-conformist career woman, she never told Worf because he would demand they take the traditional oath of union.
Their later encounters came during her special diplomatic missions, during Duras' poisoning death of Chancellor K'mpec and while diffusing the emergence of a Klingon pre-Alliance sleeper ship in 2365 that threatened to attack unsuspecting Federation outposts. Inheriting a scathing sense of humor, she had kept her Klingon side's monstrous temper under tight control.
After K'Ehleyr's murder in his arms, with Alexander watching, Worf did not easily acknowledged the guilt about the anger he felt toward his onetime mate because she had not told him about their son until he arrived; the boy was affected by his mother's oft-voiced view that 'Klingons had a lot of dumb ideas about honor.'