All of the characters in your list represent some aspect of life.
but the character that made the whole story so tragic is Ennis' father.
He represents the severe hate that is the crux of this story.
Joe Aguirre - fearful of his reputation if he hires them guys
John Twist - weird and defeated, Mrs Twist conquered him and raised a gorgeous son.
L.D. Newsome - money MAN - football + money = manliness
Ennis' Dad - EVIL
K.E. del Mar - I am sure K.E. is hurt just as much as Ennis by the irrigation ditch, how could we fault him??
Cassie Cartwright - wants to have a good man - more power to her.
Jimbo, the Rodeo Clown - represents that fear that a man has, even if he is straight. he is cute, in spite of it all.
Mr Del Mar in Sage, Wyoming gets my bad guy vote in this movie.
second on my list would be Jimbo, evn if he is physically cute.
Morey