The relationship through which we most come to know Lily is her relationship to her son Harry. For the first four books of the series, she is all but defined by her role as a mother, her love for her son, and her decision to sacrifice her life in the hopes of saving his.
The connection between Harry and Lily is often emphasized by characters’ frequent reference to the fact that Harry has inherited Lily’s eyes. But Lily’s presence in Harry’s life is not limited to the physical features he inherited from her. The primary tenet of their relationship is the fact that Lily died in order to save Harry. As the younger Lily offered herself as a substitute for Snape when others scapegoated him, here she does the same for Harry. The difference, though, is that while the scapegoating of Snape did not entail physical death, the scapegoating of Harry does. Lily’s death at the hands of Voldemort is her final, culminating act of maternal compassion.
Lily’s innocence at the time of her death, the fact that she would have survived had she allowed her son to be killed, is reiterated by Voldemort, starting in Sorcerer’s Stone, when he tells Harry, “. . . but your mother needn’t have died . . . she was trying to protect you . . . “(SS 294). Unlike James, who had been marked for death along with Harry, Voldemort had agreed to spare Lily. This much is made clear in Deathly Hallows when Voldemort approaches Lily, thinking, “as long as she was sensible, she, at least, had nothing to fear…” (DH, 344). When Lily steps between Voldemort and Harry, then, she takes on Harry’s assumed guilt and accepts the punishment for it. She even acknowledges this directly, as she pleads, “Not Harry, please no take me, kill me instead–“ (DH 344).
We get an explicit example of Lily using her body as a shield for her son in the moments leading to her death:
“At the sight of him, she dropped her son into the crib behind her and threw her arms wide, as if this would help, as if in shielding him from sight she hoped to be chosen instead” (DH 344).
Moreover, the shield Lily offers Harry transcends her death, and Lily’s final effort to save her son results in the lingering protection that would go on to save Harry countless times against Voldemort. This protection, while not felt physically, has a presence throughout the whole of the series, from the moment Harry first learns of it at the end of Sorcerer’s Stone. He asks Dumbledore why Voldemort could not touch him and Dumbledore replies:
“Your mother died to save you. If there is one thing Voldemort cannot understand, it is love. He didn’t realize that love as powerful as your mother’s love for you leaves its own mark. Not a scar, no visible sign . . . to have been loved so deeply, even though the person who loved us is gone, will give us some protection forever. It is in your very skin. Quirrell, full of hatred, greed, and ambition, sharing his soul with Voldemort, could not touch you for this reason. It was agony to touch a person marked by something so good” (SS 299).
For Harry, Lily is not only a parent but also a continuing example of maternal compassion and its power. She becomes a model which he comes to consciously imitate as he grows throughout the series; it is through her that he understands the potency of compassion and mercy and learns to extend them towards others himself.