Given the way that the novel is constructed, you could pick almost any character and show how that character has affected August.
I would think that one of the most direct approaches to this would be to choose Auggie's mother. From the most literal perspective, she has given birth to him. Another way she impacts Auggie is that she reminds him of his inherent, internal beauty. Auggie loves to hear the story of his birth because it always ends with his mother looking into his eyes and not dwelling on the condition of his face. This affects Auggie in a positive way because it offers him hope even while he is in a negative setting.
Auggie's mother's constant defense of him and tireless advocacy for him is what enables him to endure and eventually triumph over the negative, challenging conditions of his life.
In these ways, I think that you might be able to construct a narrative in which Auggie's mother has acted in a way that has significantly impacted him. Her deeds, the actions in defense and in honor of her child, are her monuments to Auggie. These monuments are some of the reasons that he is able to experience the growth that he does. The "monumental" importance she has for him is representative of why she can be seen as one of the most important characters in the narrative.