Zilpah's that friend who claims she can control the wind and predict the future. You know, the one who reads your palm and tells you you'll have five children, a pool in your backyard, and a rich husband. Zilpah's the spiritual daughter of Laban.
When the sisters are informed that they are leaving Mesopotamia, Zilpah says this:
"I cannot leave the holy tree, which is the source of my power. Or the bamah, which is soaked in my offerings, How will the gods know where I am if I am not here to serve them? Who will protect me? Sisters, we will be beset by demons." (2.2.7)
Uhh, right. Demons. Zilpah's religiosity pretty much defines her character: she's the auntie Dinah will go to for spiritual advice. Basically, Zilpah's the opposite of Leah. Remember what Leah says about the mountains? "'The Mountains will protect us against winds,' said Leah, with reason" (2.6.11). Well, let's take a look at what Zilpah says: "'Mountains are where heavens meet earth,' said Zilpah, satisfied that she would find inspiration" (2.6.10).
Where Leah is grounded in reason, Zilpah is lifted by faith.
Zilpah's also different from her sisters because of her sexuality. Leah, Rachel, and Bilhah are often driven by sex and lust—which means they're also often blinded by it. Zilpah, on the other hand, seeks pleasure in her spirituality, which makes her particularly privy to her sisters' lust: "Zilpah saw lust everywhere she looked. To her, the whole word suddenly seemed damp with longing" (1.2.18).
Though Leah might be the voice of reason, Zilpah's the one who can sense and understand people's passions and internal lives. So we shouldn't just pass off Zilpah as a cuckoo who sees dead people. She's just not your average woman. In fact, she's most likely asexual, as sex isn't pleasurable or desirable for her. Bearing a child is all she cares about: "Zilpah took no pleasure from Jacob's touch. 'I did what was required of me,' she said, with such a tone that no one dared ask her to say more" (1.3.35).
One of these sisters is not the like the others, and that sister is Zilpah.