At first, August (Auggie) doesn’t describe what he looks like. In the opening pages, regarding his appearance, he tells the reader, “Whatever you’re thinking, it’s probably worse.” He describes what his mom looks like (she’s “beautiful”), he describes what his dad looks like (he’s “handsome), and he describes what his sister looks like (she’s “pretty”), but he refrains from describing himself.
Auggie’s initial absence of a description is because he’s not confident about the way he looks. Due to myriad medical issues, August’s face does not like other people’s faces, so it’s not something that he wants to highlight.
However, as Auggie’s story unfolds, descriptions of him slip out. In the “Lamb to the Slaughter” chapter, Julian, the bully, asks about the braid that August has. It’s revealed that August has this tiny braid in the back of his hair because of Star Wars. It’s the kind of braid that a Padawan (a Jedi apprentice) possesses.
Finally, in the “Padawan” chapter, August breaks down and describes himself. After cutting off his Padawan braid (likely because of Julian’s snarky Darth Sidious comment), there’s commotion. His sister can’t believe he cut it off, and his parents are concerned.
Near the end of the chapter, as his mom reads out loud from The Hobbit, August starts to cry. Now, August describes himself. He calls himself “ugly.” He says his eyes go down too far, his cheeks look “punched in,” and his mouth resembles a tortoise’s mouth.