Nellie Owens was a year younger than Laura, and had a younger brother named Willie, just as the character of Nellie did in the books. Her parents, William and Margaret Owens, did run a local mercantile in Walnut Grove. Nellie is the primary basis for Nellie Oleson in On the Banks of Plum Creek.
Genevieve Masters was the spoiled daughter of Laura's teacher in Walnut Grove. Genny wore pretty clothes, and had beautiful golden ringlets, just as "Nellie Oleson" did. Nellie Owens and Genny Masters were in a constant battle to rule the playground, and all the younger girls were forced to take sides. Laura, however, refused and soon became the leader of them all.
Genny's family moved to De Smet not long after the Ingalls family, but the Owens family did not. Therefore, the Nellie of Little Town on the Prairie is mainly Genny Masters.
The third girl making up the character of Nellie Oleson was Stella Gilbert, a poor girl who lived on a claim near De Smet. Though poor, she was beautiful, and she had her eye on Almanzo. She did manage to beguile him into taking her on several buggy rides with him, told of in These Happy Golden Years, but when Almanzo was made aware of her shallowness, the rides stopped.
Not much is known about what happened to Genny and Stella in later life; Laura herself was uncertain. Nellie Owens moved to California, and then on to Oregon, where she married Henry Kirry and had three children, Zola, Lloyd, and Leslie. Her brother Willie went blind from a firecracker explosion, attended a school for the blind, married, and also had three children. Laura wrote to some fans that Nellie moved to Louisiana, and to others that she returned to New York. It is possible that she was referring to Genny Masters and Stella Gilbert in these letters.