Macabéa's aunt will slap you and then say she did it for your own good. She's that kind of person. Sometimes Macabéa recalls how her spinster aunt would beat her and get a sick enjoyment out of it:
Her aunt would use her knuckles to rap that head of skin and bones which suffered from a calcium deficiency. She would thrash the girl not only because she derived some sensuous pleasure from thrashing her—the old girl found the idea of sexual intercourse so disgusting that she had never married—but also because she considered it her duty to see that the girl did not finish up like many another girl in Maceió standing on street corners with a lit cigarette waiting to pick up a man. (3.62)
Okay, so she's a crazy lady who apparently had issues with intimacy and sex and took out her frustrations on her defenseless dependent. Now we know where Macabéa's sexual hang-ups come from, at least.
The aunt has another trick up her punishment-and-abuse sleeve: deprivation. She takes away Macabéa's favorite dessert and she denies her one of the only things she's every wanted: a dog.
The thing is, we don't really know much about the aunt as a character in her own right. What she does is fill in Macabéa's backstory. There are a lot of reason, the narrator suggests, that Macabéa's life is so hard—but this aunt is one of them. As we learn, "Her aunt's constant reproaches had taught [Macabéa] how to keep her head lowered" (3.64)—and to expect nothing but abuse in intimate relationships.