Isabel's little sister doesn't say much, but Ruth's vulnerability and innocence are the driving force behind Isabel's desire to seek freedom. Having lost both of their parents, Ruth is all Isabel has left, and she feels a responsibility to Momma to care for her at all costs.
Ruth's sensitivity is one of the most striking aspects of her character. In Isabel's words, Ruth is "simple-minded and prone to fits" (3.9), which is an 18th-century way of saying that she's developmentally delayed and epileptic. She's often given to seizures as a result of heightened emotion, stress, or loud noises, and becomes emotionally upset very easily.
So what's the problem? Mental and physical handicaps didn't go over too well in the world of slavery. Want proof? Check out Madam Lockton's reaction the first time she sees Ruth have a seizure—she calls Ruth "the devil" (15.1). Now we, as astute, modern-day readers know that a little girl who likes pretty horses and cornhusks dolls probably isn't the devil, but Ruth's differences serve as an illustration of how impairments of any kind could make slaves' already difficult lives even worse.
What's really weird about Ruth is that she almost seems more present in the story after Madam sends her to the Lockton estate in Charleston. This is probably because once she's gone, Isabel replays memories of her, struggling to hold onto whatever little she has left of her sister. When Isabel wakes up to find the sheets frozen on the wash line in the snow, her first thoughts are about her sister:
Ruth would love this. If we were free and at home in Rhode Island and they were our sheets and our laundry lines and our snow, she'd dance like an angel. (41.34)
For Isabel, Ruth is inextricably attached to her desire for freedom. When Isabel learns that Ruth is only in South Carolina and not the West Indies, she becomes even more determined to seek both her freedom and her sister. We've got a hunch that we haven't seen the last of Ruth as this saga continues.