Ahmed makes no bones about her love for her nanny: after all, she spends her childhood terrified that Nanny would die. This is normal, sure…but Ahmed seems more concerned about Nanny kicking the bucket than her parents.
It makes sense, though. Nanny is her #1 companion and parental figure. She takes care of Ahmed day and night and saves her from total boredom and isolation.
She also admires Nanny's morals and resolve. And here's something special: even though Nanny is a Christian, she's extra careful not to influence Ahmed's religious beliefs. Nanny's approach to religion and spirituality feels like a warm hug, embracing everyone who believes in one God:
...she always spoke of Islam with the same respect that she spoke of her faith. Moreover, though her Bible included pictures of Jesus and Mary, when she spoke of her beliefs she spoke only of God, Le Bon Dieu, and God was the same, she said, whatever religion you were, whether Muslim, Christian, or Jewish. (62-63)
In this respect, Nanny agrees with Ahmed's mother. Though the two of them are pretty much archenemies, they do share the same moral principles. In fact, Ahmed thinks that their "professionalism" about child-rearing made her life a whole lot easier as a kid.
But, like any good Mary Poppins figure, Nanny has her quirks. She really, really likes men. Don't get the wrong idea—she's super chaste. But she also kisses up to Grandfather, prefers the boys in the family, and hates on Mother because she's a woman with the ability to fire her.
It somehow offends her sensibilities to think of a woman in a position of power. Ahmed sees this as a fault in Nanny's character since it compromises her high moral standards.
And then there are the kitties. Although Nanny can be a sensitive soul (she cries over Lassie), she's also kind of a devil:
Nanny [...] also sometimes drowned kittens the moment they were born. Not, as a rule, those born under the stairs, who belonged to special cats, but those she might find elsewhere, the litter of the garden. (58)
It's a weird kind of elitism that jibes with Nanny's philosophy of life, which features a clear-cut morality, an awe of the privileged class, and a hatred of weakness (moral or otherwise). It's Nanny's sense of morality (and perhaps a little streak of cruelty) that prompts her to tell Ahmed of Mother's ultimate maternal betrayal (i.e., trying to end her last two pregnancies).
This revelation haunts Ahmed for the rest of her life and makes her doubt the sincerity of her mom's love. Well done, Nanny. It's a sign that she's a stand-out character in Ahmed's life story, and that her actions—murdered kitties included—made a serious and lasting impression on a little girl's mind.