How do the characters in "speaking of courage" story cycle, principally Bowker, Cross, O'Brien, and "the boy" deal with their feelings of responsibility and guilt over the death of Kiowa? What is Kathleens role? Who might she represent?

I can't go through each character for you but hopefully this helps:

This story contrasts with the other chapters in the collection because of its almost utter lack of dialogue. The story is technically in third person, but is told almost entirely from Bowker’s perspective. Bowker, broken, alone with his thoughts, is a symbol of the many veterans who could not adjust after returning home to the US. Vietnam was a deeply unpopular war, and many of the veterans felt dishonored for having fought in it. Kiowa’s death is the climax, the organizing tragedy that comes up over and over again throughout the book.

This intensive look at Bowker sets him up as a foil for the narrator, O’Brien. There are many doubles in this book: the Vietnamese girl and Kathleen, Jensen and Strunk, Rat Kiley and Jorgenson. In most cases they are each others’ opposites, but are still linked by some sort of commonality. The young Bowker and young O’Brien were on parallel tracks that completely diverged after the war. Only then did the two become opposites: the successful young man and the aimless, damaged vet. O’Brien suggests a remaining similarity, though, by writing about how shaken he was by hearing about Bowker’s experience. O’Brien recognizes that their paths could just as easily have remained the same. This is one of the only stories in the book where O’Brien indicates that he is lucky.