She might be tough and strict, but Becky, Madam's housekeeper, is one of the few sources of comfort Isabel has during her time with the Locktons. Although she believes the most important rule of order in the house is to "do what Madam says" (7.16), she's not above showing dignity and kindness to Isabel and Ruth and treating them like—of all things—human beings. She's also their primary educator on the various rules of Life with the Locktons, a.k.a. How Not to Get Beaten and Potentially Die.
Need proof? Check out the scene where Ruth has her seizure in the kitchen. Even when Madam kicks up a fuss about demon possession, Becky is quick to point out how fast Ruth recovers. "Look there," she says. "The little one is already back at her chores. And she did a fine job sweeping earlier" (15.29). When it comes to keeping the peace around the hot-tempered Madam Lockton, Becky's a pro.
Becky's ability to manage situations like this also comes from another key element of her character: knowledge and experience dealing with the Locktons' dysfunctional behavior. She's able, for example, to warn Isabel about the consequences of crossing Madam—Becky once saw Madam react to a slave who talked back by "beating her regular-like" (8.29) until her arm was withered.
She also knows that even when the Locktons' fights get so out of hand that they're screaming and breaking things, it's best not to interfere, as talking to Lockton will "only fire him up more" (13.17). So though life at the Locktons is definitely rough, it's clear that without Becky things could be a whole lot harder for Isabel and Ruth.