Sophie Mol is a pivotal character in the novel, but we don't really get to know her very well. When you think about it, most of the detail we get about Sophie comes from what people expect her to be or from how people remember external qualities about her. In many ways, she's more of a type than an actual person.
Our understanding of Sophie Mol mainly comes through the ways in which the other characters – particularly Estha and Rahel – perceive her. We do know that she is the half-English, half-Indian daughter of Chacko and Margaret Kochamma. She's always wearing yellow bellbottom pants. She has a go-go bag that she loves to carry around. She misses Joe, her deceased stepdad, terribly. She has light-colored eyes and dark hair. She can tell that she's different from Rahel and Estha because they're fully Indian while she is half-white.
Because we learn about Sophie Mol mostly through Estha and Rahel's eyes, it's easy to dismiss her as the spoiled brat you love to hate. The twins don't particularly like her because she makes them feel inferior. Other members of the family, particularly Baby Kochamma, constantly compare them to Sophie in ways that make her seem better. We all have that person in our lives – whether it's a glamorous cousin, an older sibling, or a popular kid in school – who makes us feel like we just don't measure up.
The thing about Sophie Mol, though, is that, as much as we're inclined to dislike her, she isn't actually all that bad. It's easy to see how Rahel and Estha dislike her based on their preconceptions about her rather than who she really is. Unlike them, we get the chance to see what Sophie is like on her own. At the end of Chapter 13, we see that Sophie actually wants to be friends with her cousins, and that she's the one who feels left out. She tries to win them over the best way she knows how: she gathers up presents to give them:
Sophie Mol put the presents into her go-go bag, and went forth into the world. To drive a hard bargain. To negotiate a friendship. (13.185)
There are other ways that Sophie tries to get the twins to like her. She insults Chacko and Baby Kochamma to win their approval, and she begs to tag along with them when they decide to run away. This decision, of course, proves fatal for her. As much as the twins dislike her, and as much as we're initially inclined to follow suit, in the end we see a very human, sensitive, and fundamentally lonely little girl in Sophie Mol.