Dark Link. He is the very image of Link himself, but evil. He is probably one of the most symbolic characters in the series, because he represents that Link must conquer himself if he is a true hero, just like how we must conquer ourselves for our well being (such as our fear, anxiety, self doubt, anger, hate, etc). By taking this dark side of Link and molding him into an actual evil entity, Link must fight himself.
Dark Link appeared as the final boss in AOF and as a midway boss in the Water Temple in OoT. I know he's made other appearances in other games, too