A hero is selfless
To be selfless, means being able to put others above yourself. A true hero must be able to set aside all personal quandaries to focus on the matter at hand. Being able to think about a total stranger and put their needs above your own takes character. Human's in general have a natural selfishness to them, it isn't a bad thing. It's important to take care of yourself and watch out for you, though, taking it a step further and focusing on another person other than yourself, gives you a quality that is imperative in the means of being a hero.