In chapters 1 and 2, we learn about Ralph's appearance: he is an athletic, attractive, robust English boy, the son of a naval officer. His traditionally English good looks suggest that he is the proper leader of the other boys.
We learn he is a "fair," or light-haired, boy from the Home Counties in the south of England, a prosperous area. When he and Piggy come to the water at the beach, Ralph is excited and rips off his heavy black shoes and schoolboy clothes. We then find out he is twelve years old with the shoulders of a boxer and the grace and slimness of a boy who is no longer a child but not quite an adolescent. He has a "mild" mouth and eyes, as described on page 11 of my edition of the novel, in chapter 1:
He was old enough, twelve years and a few months, to have lost the prominent tummy of childhood and not yet old enough for adolescence to have made him awkward. You could see now that he might make a boxer, as far as width and heaviness of shoulders went, but there was a mildness about his mouth and eyes that proclaimed no devil.
He is able to caper around and stand on his head with ease. He can also swim easily in the lagoon.
It is Ralph's "stillness," or sense of composure, his "size," and his general good looks that cause the other boys to elect him as their "chief" early on. Jack wants to be their leader, but he has a thin face and is less physically appealing than his rival.