Harry Potter and the Half-Blood Prince
Merope is Voldemort's mother. Merope suffers much in life, the likes of which we see with the help of memories that Dumbledore has procured. In her young years, she lives with her father, Marvolo, and brother, Morfin, both of whom are abusive. She has no love and no pleasure in her life, and her magic is severely weakened on account of her living situation.
Though her family is staunchly anti-Muggle, she falls in love with the handsome Tom Riddle, a Muggle who lives in the nearby manor house. After casting a love spell on Tom, the two run away and conceive a child. Loving her husband so much, Merope releases him from the spell, hoping that he might love her still. He does not. Tom immediately leaves her, telling the world that he had been tricked into marrying her. Merope grieves heavily after this.
The biggest choice Merope makes in her life is to not use her magic to save her own life. She chooses to die rather than to be a mother to Tom Riddle, Jr. (Voldemort). After giving birth to him at an orphanage, Merope dies. Merope's story, life, and ultimate choice play an essential part in who Voldemort is.