Just to be able to put up with Zeena even knowing all that she was doing to him financially, psychologically, and socially, tells anyone that Ethan's level of tolerance is his strongest characteristic. He seems to be able to accept the many disappointments he has had in life. However, that very well may be simply pessimism on his part, and the expectation of the worst.
Other than that it is very hard for me to say that he is a strong man in any other scenario because he never learned to put his foot down and take his role as the head of the household. He did stand up to Zeena maybe once in the entire story, but we know that his last escape with Mattie was nothing but a very cowardly way to leave Zeena without confrontation and opting for death as a better option. That does not say much about a man's strength of character.