Who is the main character of the story "Where Have You Gone, Charming Billy" by Tim O'Brien?

The main character of the Tim O'Brien short story "Where Have You Gone, Charming Billy" is Private First Class Paul Berlin. On the first day he is fighting in Vietnam, he watches Billy Boy Watkins step on a mine and lose his foot. Though his wound is not fatal, Billy Boy becomes so agitated about dying that he has a heart attack that kills him.

Thoughts about Billy Boy and the constant refrain "Where have you gone, Billy Boy, Billy Boy, Oh, where have you gone, charming Billy?" keep going through Paul's mind as he recounts the day in a stream-of-consciousness style. Paul is able to get through the day by imagining that his platoon will reach the refreshing sea the next day, and then he imagines what his father would say to him. When he lies down next to a soldier named Toby, Paul dissolves into nervous giggling that he can't stop as a reaction to the day's events. He is filled with dread and fear about the war. He tells himself that he will later regard the whole story of Billy Boy as a joke. However, as the story ends, Paul continues to be afraid, even when he sees the sea.