Roberta runs the Peacock Tattoo Emporium in Grandma's neighborhood, and Dolores hangs out with her to "smoke and swear" (4.70) about life because she doesn't want to do that with her own mother. Honestly, Roberta isn't telling Dolores anything her mother isn't telling her at this point—like, say, "Don't you ever become some man's personal toilet the way I did" (4.74)—but Dolores actually listens to Roberta because she's at an age where she won't listen to her mother, no matter what her mother says.
After Dolores is raped, Roberta is the one she tells. She feels safe with her, maybe because Roberta survived her own abusive relationship: "One day he'd love me right and the next day he'd slam me against the wall" (4.125), she says.
We don't see Roberta for a few hundred pages after this until she comes back into Dolores's life after Grandma dies, becoming one of her only female friends. She encourages Dolores to look on the bright side of life—her mantra is "Life's a polka and don't you forget it!" (25.209)—and by this point, she's thrown in the tattoo towel and she's the Polka princess on the radio. Livin' the dream, if you will.
Roberta is focused on mobility. Upward mobility, that is. She wants Dolores to get rid of her TV, which motivates Dolores to sit on her butt and do nothing, and get a car so she'll go out and do something. Anything.
Eventually, Roberta's get-up-and-go attitude sinks in for Dolores, and they become an unstoppable pair, like Thelma and Louise, delivering Chinese food for extra cash and going to see whales. As Roberta gets older, she kind of becomes the child that Dolores never had—and never will have. Roberta once took care of her, and now Dolores will take care of Roberta. In some ways, this is the best family Dolores ever has.