The first two boys William Golding introduces in chapter one of Lord of the Flies are Ralph and Piggy. Before we know their names, they are known to us only as "the boy with the fair hair" (Ralph) and "the fat boy" (Piggy).
Ralph appears first, and he is walking energetically through the jungle foliage. Piggy sees him but is struggling to keep up because he is overweight, has asthma, and has to stop because he is suffering some diarrhea from eating too much fruit. Piggy calls out to him, but Ralph just keeps walking, though Piggy eventually does catch up.
Ralph is a natural athlete and a good swimmer; Piggy is neither. Ralph is a very physical boy; for example, when Ralph realizes that there are no grownups on the island, he immediately stands on his head. Everything Ralph does in this chapter suggests movement. He is constantly diving, leaping, jumping, climbing, or hurrying. In contrast, Piggy is always puffing hard with exertion as he tries to catch up with Ralph.
Piggy is not the kind of boys other boys naturally like very much. He is fat because his parents are dead and his "auntie" continually gives him sweets. He wears thick glasses and a windbreaker, suffers from asthma attacks (which do nothing but annoy everyone else), and wants to be sensible about their plight. Ralph cares nothing about being sensible, at least right now, and he would ditch Piggy in an instant if he could.
Later their relationship will change, and Ralph will come to appreciate Piggy's contribution to his leadership. In the beginning, though, they are nothing alike and do not work together in any way.