"Personalities" implies social responses and one's view of one's own worth.
Ender has grown up knowing that his birth order puts him in a socially distasteful and precarious position and as a young man his personality adjusts to that unchangeable condition, making him a socially wary human being, a personality trait that serves him well in the military training school. Valentine, on the other hand, has been tested for the school and found sufficiently intelligent but not exactly "right" for the social responsibilities the training anticipates. Consequently, she has become a protector and nurturer (without making a stereotypical "feminine" character out of her). Their mutual experiences, first as children and later on Ender's furlough, demonstrate the differences in the ways each approaches the social and interpersonal responses to a sometimes hostile world. It is tempting to reduce their differences to their gender difference, but the author avoids that oversimplification, especial in Ender's foresight and "humanistic" response at the close of the novel.