Just Mariam and Laila's friendship in general. Although Mariam is initially cold towards her (and you can't really blame her, because Laila really is her replacement in Rasheed's eyes), they eventually bond over late-night tea and turn to each other for comfort and support when faced with their abusive husband. Mariam, who cannot have children, becomes a second mother to Laila's, not to mention becoming a mother figure to Laila herself (whose own mother neglected her in her depression over her sons' deaths). Much of the book is devoted to the camaraderie and love between Mariam and Laila and how they complete the other. This devotion culminates at the end of the novel, when Mariam kills Rasheed who was savagely beating and about to murder Laila for meeting with Tariq, her childhood sweetheart whom Rasheed had led her to believe was dead. What's more, Mariam takes the full blame for killing him and turns herself in, allowing Laila, Tariq, and the children to escape the country and live in peace while she is executed. Although heartbroken, Laila still carries Mariam in her heart and visits Mariam's childhood home in the end, imagining how her life shaped her to become such a beautiful person.
Laila finding Tariq alive after being told he had died.
Zalmai slowly accepting Tariq as a father-figure.
Tariq comforting Zalmai.
Jalil's letter to Mariam, given to Mullah Faizullah for safekeeping, along with a video of Pinocchio and whatever money he can spare to her.
The very last sentence of the novel: "But the game involves only male names. Because, if it's a girl, Laila has already named her."