Agatha is the most important character who never actually makes a real-time appearance in the book. She disappears before the first page, and all we get from her in the end is a letter. Everything we know about Agatha, then, we learn through Georgie's flashbacks, and while Georgie is often angry with Agatha and misunderstands her, she also idolizes her older sister.
Agatha's driving force appears to be her interest in nature and her desire to formally study the natural world. First she begs for tuition money for the University of Wisconsin, but when it doesn't look like that's going to happen, she starts going out with Mr. Olmstead, who shares her interests and has a library full of great books. Agatha's a tricky one, though, because while at first it seems like she's using Mr. Olmstead, it turns out she actually does like him like that. Given how much Georgie admires her, we’re glad Agatha's a decent human being.
Georgie describes Agatha as a feather: "Agatha? She was a feather. She pushed higher, farther always" (3.2). In the end, Georgie says, "Feather by feather, she had made her way" (23.37), in admiration at the way Agatha has managed to live her own life despite the obstacles. You go, girl.