Barnes
Barnes is the Staff Sergeant for Taylor's unit in Vietnam. He is a symbol of chaos that is unleashed in war as his morality is non-existent on the battlefield as he orders rape, murder and kills his own soldiers. All of this for his own sense of justice and to cover up his wrongdoings.
Elias
Sergeant Elias represents the faction of soldiers who are there to do their job. And while he may seem strange to some for partaking in drugs (smoking a joint with his troops), he has a strong sense of morality and to his duty as a soldier.
A Sergeant's Death
Barnes has lied to Taylor about Elias being killed by the enemy. Taylor sees Elias has been shot and is running from the Vietnamese soldiers as they chopper out of the area. Elias is killed and this is a symbol that the word of America is tainted, just as Barnes is tainted in his attempt to explain-away the missing Sergeant.
Youth
Lieutenant Wolfe is the official leader of Taylor's infantry platoon but the soldiers truly follow the older, seasoned soldiers Barnes and Elias. This is a symbol that the troops don't want to be led by some young man who's never seen battle himself. They will only trust someone who's put their life on the line before, men like Barnes and Elias.
Francis
Francis and Taylor have been wounded already in the fighting and during the end of the film we see that Taylor is once more been hurt. Francis, on the other hand, has not been but ironically he creates a self-inflicted wound upon himself by stabbing his own leg in order to be sent home.
