I've always thought of a hero as someone who risks/sacrifices themselves to protect/save/rescue someone else.
he·ro [heer-oh] Show IPA
noun, plural he·roes; for 5 also he·ros.
1.
a man of distinguished courage
or ability, admired for his brave deeds and noble qualities.
2.
a person who, in the opinion of others, has heroic qualities or has performed a heroic act and is regarded as a model or ideal: He was a local hero when he saved the drowning child.
3.
the principal male character in a story, play, film, etc.
4.
Classical Mythology .
a.
a being of godlike prowess and beneficence who often came to be honored as a divinity.
b.
(in the Homeric period) a warrior-chieftain of special strength, courage, or ability.
c.
(in later antiquity) an immortal being; demigod.