In Lord of the Flies, William Golding first characterizes Piggy's vulnerability through his obesity, his name, and his asthma.
One of Piggy's most vulnerable moments happens early on in the story. The moment Piggy meets Ralph in chapter 1, he takes Ralph to be a civil, trustworthy person. Hence, Piggy makes the mistake of taking Ralph into his confidence by saying, "I don't care what they call me ... so long as they don't call me what they used to call me at school" and proceeds to inform Ralph that he was called "Piggy." However, instead of receiving the consideration he was expecting, Ralph proceeds to "[shriek] with laughter," dance on the beach, chant the name Piggy, and mock him in any possible manner. Ralph even later insults Piggy by informing the group his name is Piggy. And, naturally, Piggy's name is a result of his obesity, which is a result of being prevented from participating in physical activity due to his asthma. Hence, this very early scene serves to paint Piggy as a very vulnerable character.
Yet, despite his vulnerability, Piggy emerges as a devoted leader, maybe even as a stronger leader than Ralph. Piggy's leadership abilities emerge as he acts as an adviser to Ralph and acts in devotion to maintaining law and order. Piggy particularly demonstrates his devotion to law and order by being the first to suggest using the conch and, moments before his murder, grabbing the conch and demanding of the boys to choose, "Which is better, law and rescue, or hunting and breaking things up?" (ch. 11).