The biker, "Al," is symbolic of consequences for actions.  We're assuming that "Al" is the dead body in the murky, trash-filled lake.  His bike is parked at the edge of it and the boys do not know whose bike it is or where the person is that it belongs to.  However, after the narrator is forced to go into the lake to hide for fear of losing his life, he happens to run into the dead body that is floating in the lake.  He describes in detail how the body feels and this is when he has his epiphany (moment of enlightenment).  He realizes that he and his friends have been trying to be something they aren't..."bad" troublemakers.  The narrator knows that this isn't his true nature.  He and his friends are doing so because it is cool and they want to be seen as being cool.  They quickly realize, however, that almost raping a young woman and beating someone up with a tire iron are NOT cool things to do and that they all could have been killed because of their irresponsible and reprehensible actions.  Because running into the body of the biker initiates the narrator's epiphany, we can see this event as symbolic of the possible consequences of his and his friends' actions.