We know Ammu best as Estha and Rahel's mother, but it's important to look at her early life when we examine her character. Ammu isn't just a mom; she's also Pappachi and Mammachi's daughter and Chacko's younger sister. Through a number of flashbacks in the novel, we get a good sense of how her past helped shape who she is as an adult.
While Chacko, as the son of the family, is proudly sent off to school and eventually to Oxford to study as a Rhodes Scholar, Ammu doesn't get the same kind of treatment. In fact, it seems like she doesn't have all that many options. Life at her parents' house is tough. Pappachi is kind of a drunk jerk, terrorizing Mammachi and Ammu. Feeling like there's nothing left for her in Ayemenem but to wait around for a husband, Ammu gets permission to go to Calcutta for a summer. She meets and marries Baba, which turns out to be a bad choice.
Ammu divorces Baba and moves back to Ayemenem when the twins are toddlers. Ammu is both a strict and loving mother. What's interesting about her is her resistance to social norms. She doesn't feel like she needs to be ashamed of her divorce. Instead, she feels like she wasted her best years. This quality sets her apart from the other women of the household, who are totally preoccupied with looking better than others in society.
Ammu doesn't think too much of social rank. In fact, she's kind of proud that Velutha was spotted at the communist march, figuring that they both find society's norms oppressive and wrong. This doesn't mean Ammu doesn't care about appearances altogether; she wants her kids to behave well so that everyone can see that a woman on her own can be both independent and a good mother.
Ammu's affair with Velutha is a great escape for her, and we can tell that she has really strong feelings for him, even though she knows their love can never be public. In a way, it's their love that brings about both of their downfalls. Baby Kochamma wouldn't have had to make up stories about Velutha to the police if they hadn't had an affair. Ammu's life after Velutha's death is unimaginably bad. She's lost the only man she ever loved, and she's separated from her kids. She dies alone, likely of tuberculosis, in a dirty hotel room. Still, when she is cremated, Rahel doesn't remember her as the disgusting, hacking, slightly loony woman that she ultimately becomes. Ammu is remembered as lovely and, most importantly, loving.