William "Billy" Hayes was an orphan boy adopted by Burt and Ellen Hayes and their young daughter, Greta. When he was first adopted, Greta saw prophetic visions of his evil nature, but was calmed when Billy swore to protect her from the bad things of the world. However, Billy displayed signs of violence as a child. When he was 11, he sliced Burt's neck with a knife, missing his jugular by an inch.
When Billy entered his teenage years, he was approached by the demon Buzz, who offered him immense power. To gain this power, Billy put on tribal face paint and sacrificed Greta by dropping a tape-deck into her bathtub. Billy then took on the name Harm, a super villain in-training determined to become the world's greatest murderer.
Harm made his first appearance in Young Justice #4, attacking Arrowette and stabbing one of her own arrows through her shoulder. The attack on the young superheroine prompted her friends in Young Justice to seek out Harm, but he was abel to defeat them all easily.
Harm then captured Red Tornado, reprogrammed him, and placed a bomb in his chest. He then left a clue for Young Justice and attempted to kidnap Pope John Paul II. Young Justice is able to rescue the pontiff and remove the bomb from Red Tornado before it causes any damage, but Harm escapes during the fight.
Harm's parents, Burt and Ellen, lived in perpetual fear of their adopted 17-year-old son. One day, Burt promised Ellen he'd "handle things." While she was at her mother's for the weekend, Burt shot and killed Harm.
Harm later returned in Young Justice #14. During the "Day of Judgment" crossover event, Hell itself froze over, allowing the dead Harm to return to the living world through the form of Secret. As before, Harm was able to easily defeat the members of Young Justice. But this time, Secret realized that Harm is actually her brother, Billy, who had killed her years ago. She launched herself at Harm and propelled them both over a cliff. Impulse searched the site at superspeed, and was unable to find their bodies — but Secret did eventually return.
Harm's final appearance was in Young Justice #52. Burt Hayes was sent to prison for murdering his adopted son. While awaiting his death sentence, Harm took over his body and led Secret to believe her father didn't love her. Secret then enlisted the help of Slobo to break her father out of prison. She then joined Darkseid, and took her father with her to Apokolips. While there, Harm forced Burt to jump into a fire pit, killing them both.
- Main article: Harm (Young Justice)
William Hayes murdered his younger sister, Greta, in order to become "pure evil". He did this in order to become worthy of wielding the Sword of Beowulf, an ancient weapon he subsequently stole from a museum. Overhearing the incantation to activate the weapon's powers, he speaks it, quickly killing a room full of police officers and the museum curator, taking the name of "Harm". Later the same night, he notices Artemis and Zatanna fighting crime around the city. Eventually, he confronts the two, pursuing them until they happen upon Harm's own house.
Knocking the two unconscious, he questions Artemis on how they had found his home. She remarks that his partner led them there, only for him to say he had no partner. Zatanna and Artemis eventually free themselves, running into the backyard to discover his sister Greta's grave. He remarks about how she was the only person he loved, and that by killing her he could become "pure". Her spirit appears before him and removes the "purity" from his body, therefore making him unworthy of using the sword. After being disarmed, he attempts to strike at her, only for his attacks to pass through her body. He is then defeated by Artemis.