In Wonder, how does Via react to Auggie's deformity, and how is she affected?

In short, Olivia reacts to August's deformity by defending her little brother in public and in private. Olivia is affected by becoming truly humble. Olivia's love for her brother makes her a kind and compassionate character.

The name "Auggie" you use in your question is, of course, an affectionate nickname for August. Further, the name "Via" is the nickname for Olivia, August's older sister. August has had his deformity since birth. This deformity (which includes a severe cleft palate) affects his facial features due to differences in the skull structure. Near the beginning of Wonder, August says that he will not ever reveal what he looks like because "it's probably worse" than "whatever you're thinking." Olivia, though, describes August's deformity in detail. Olivia is not afraid of August or his deformities. Olivia loves her little brother.

First, when Olivia is very young, she defends August in front of other people by "shouting at them" if they react poorly to his deformity. Olivia is August's older sister; therefore, she feels a need to defend her little brother. Olivia's humility can be seen in that she does not mind that August is always the center of attention. As a result, Olivia is called "the most understanding little girl in the world." Olivia has always understood that because August's needs include surgery and therapy, she should never complain about her own everyday needs. Through it all, Olivia is always humble. This humility, over all, is how August's deformity affects her.

My worst day, worst fall, worst headache, worst bruise, worst cramp, worst mean thing anyone could say has always been nothing compared to what August has gone through. This isn’t me being noble, by the way: it’s just the way I know it is.

Later, Olivia's protection turns into care for August's self-worth. For example, Via is very distressed when August cuts off his little braid hanging from the back of his head. The braid, of course, is a "Star Wars thing" and "a Padawan thing." Star Wars is important to August, and that is why Olivia is upset about her brother changing his hair. At another point, Olivia is upset when August will not go to the Halloween parade because she knows it is a significant part of his personality and happiness. Olivia ends up convincing August to go to the parade.

Despite her compassionate and caring character, it is important to note that Olivia does feel a sense of relief when she goes to middle school and high school because she is fully separate from August. In fact, her feelings build and Olivia eventually blows up saying that she was "left alone my whole life." These angry feelings are fleeting, though, and Olivia goes back to being her sweet and loving self around her little brother.