True strength of character?
Willingness to sacrifice, and wisdom. Those who are willing to pay the price in blood, sweat, and tears to achieve their ends, and who are wise enough to know to create rather than destroy, create the future. A loss of either faculty brings with it travesties, selfishness, and ignorance. All other characteristics one could ascribe positively (bravery, determination, intelligence, confidence, compassion, tolerance, et cetera) are expressions of one or both of those traits, working in tandem.
Life is merely a series of choices. Learning to live with the consequences, learning to take control of your life--that's wisdom. Learning how to mold and shape your life, how to let go of pain and give of yourself to make life better--that's a willingness to sacrifice.