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Nancy is inherently caring and compassionate. Oliver reminds her of her own lost youth and innocence. At one point she, for the first time, lashes out a Fagin in Oliver's defense,
"I thieved for you when I was a child not half as old as this (pointing to Oliver). I have been in the same trade, and in the same service, for twelve years since; don’t you know it? Speak out! don't you know it?"
Nancy eventually betrays Fagin and Sikes to save Oliver, but she will not leave them, and pays her life for this decision.