An avenger is a person who avenges — someone who goes after the people who did harm, to punish them or to even the score. In older stories the avenger might be a family member seeking justice after a killing. Today the word is tightly tied to superhero films: The Avengers are a team of heroes who fight back. So students often hear avenger and picture a costume, though actually the older meaning is wider and darker: not always a hero, just someone who pays a harm back. The ending -er means 'the one who does this.'