Madame Carlota is a clairvoyant (a fortuneteller) that Macabéa visits at Gloria's suggestion. She's a voluptuous woman with shiny red lips and cheeks, "a large china doll that had better seen better days" (5.372).
Well, it actually sounds like she's seen worse days, because from the sound of it she's had an ugly past. Her whole life has been about hustling others to make ends meet (whether as a prostitute, manager of a brothel, or a fortuneteller). For example, while on the one hand she says that she was friends with the other prostitutes in the red-light district, she also says that she enjoyed the fights too. She says, "I enjoyed punching, biting, and pulling the hair of anyone who crossed me" (5.381).
She's doesn't have to do much punching anymore, but she's still a vulgar woman who seems to be quite proud of the fact that she has achieved success by taking advantage of clients and getting away with it. Notice that as she tells her story, she stuffs her face with one chocolate after another, not bothering to offer one to Macabéa: hardship hasn't exactly made her feel sympathy for the less fortunate.
The weird thing is that, as unattractive and vulgar as Madame Carlota is, she actually has a profound effect on Macabéa. It isn't until the exact moment that Madame Carlota tells her how bad her life is that Macabéa actually realizes that she's miserable. In fact, previously she "had thought of herself as being happy" (5.414). So, maybe Madame Carlota isn't so much a "fortune" teller as a present-teller.