Question

There is really no direct physical description of Jem, that I can find, in the novel. Boo's little sculpture of him is the closest we get:

The boy had on shorts, and a shock of soapy hair fell to his eyebrows. I looked up at Jem. A point of straight brown hair kicked downwards from his part. I had never noticed it before. ch 7

Google Jem, and you will see a picture of him feom the movie.