Summer was the first child born to Bert and Helen Day. Bert was deeply disappointed not to have a son, and so to earn his love, Summer spent her youth trying to excel in academia and athletics. In many ways she felt pushed into being the son her dad never had, especially after the Day's second child, Doreen, was born.
Over the years, Summer tried but never measured up to the expectations of her police officer father. The final blow came when she failed as a police cadet and he never said a word. Her frustration turning to hatred, Summer left home and joined the peace corps.
While in Africa, Summer found a native wounded by an arrow. As she began to treat the wound, he turned on her and bit her face. She later learned that the native was actually a ferocious were-hyena who would have killed her had not a fellow Corpsman shot the beast.
Back in America under a full moon, Summer turned into a living were-hyena herself and set out to create havoc in Manhattan. Her scavenging was cut short by Firestorm and she was driven off. Over time, the Hyena reappeared and devoted herself to revenge against the Nuclear Man, even teaming up with Multiplex for a short while.
On one occasion, Firestorm managed to track Hyena and discovered to his amazement that she was the older sister of his girlfriend Doreen Day. He also found Summer's diary which told of her experience in Africa. Firestorm then tracked and captured the Hyena with a minimumm of force.
Summer's parents sent her to a clinic in Arizona hoping that they would find a cure for their daughter. It was diagnosed that emotional stress triggered the change in her and so psychiatrists were called in. One, Jivan Shi, was angered at losing his status as a doctor because his qualifications were not recognised in the United States - his degree being from India. Jivan Shi manipulated Summer into becoming the Hyena again and infecting him with the "curse" so that he could gain revenge on the medical profession.
During the ensuing battle between Summer and Firestorm, Shi became a were-hyena also and ended up infecting Firestorm with the curse. Eventually back in Africa, a magician removed the curse from Firestorm, who was then able to capture both Hyenas. Medical science continued to search for a permanent cure for the Hyena affliction but this was rather redundant since a magician did remove the curse of the Hyena from Firestorm with magic. In the end, one of the two Hyenas was shot by Deadshot in Villains United in order to send a message to the Secret Society while the other Hyena was killed on Planet Salvation when a Parademon shot it through the chest.
Back on Earth, the Hyenas had mauled and bitten several people. These victims were exposed to the same Were-Hyena venom that transformed Summer and Jivan into the Hyena. The Justice League dispatched Black Canary, Vixen and Zatanna to handle this Hyena problem. The Hyena pack headed towards New Orleans to wreak havoc. Black Canary used her supersonic scream and Vixen used the animalistic skills of a lion (lions are the only creatures that hyenas are terrified of) to take down the Hyena pack. Zatanna finished the job by removing the curse of the Were-Hyena from the Hyena victims.
As Ronnie and Jason learn to cope with their new powers, their attacked by ZitherTech's most savage team, the Hyena Sqaud. The Hyena Squad consists of four soldiers who take an enhancing drug, making them feral, strong, increasing their speed and agility, and enhancing their senses. A side effect makes them giggle uncontrollably. As Ronnie and Jason fight, one of the Hyenas shoots Jason's girlfriend. Enraged, Jason unleashes his transmutation powers, transforming one soldiers armor into barbed wire, another's gun into a snake. They eventually defeat them.
Later, in the series, Multiplex kidnapps Ronnie Raymond's mom and hires Killer Frost, Black Bison, Typhoon and a Hyena. Hyena is now one of the Hyena squad members who somehow turned into a wear-hyena monster. Wether this is from Firestorm, the drug or other means is never explained. Hyena is the last enemy standing and is ready to devour Jason's girlfriend and father, when he is blasted into a street light pole by Major Force.
Hyena and his team are now seen working with the Secret Society, Plastique shown attacking Madame Xanadu and kidnapping her. Hyena and the team are later seen at the Crime Syndicate meeting. They leave and try to comfort Killer Frost (who is depressed because Firestorm is supposedly dead and he was her one hope of returning to normal) but give up and lets her sulk. Hyena and his group are later seen in Central City, tasked with destroying what remains of the city. The Rogues clash with them, deciding that they won't help the Syndicate. Hyena battles The Trickster, who simply toys with him. After chasing Trickster and dodging his explosives, Hyena manages to take a massive bite from his shoulder. He is then defeated by Trickster. Hyena is also mentioned in Forever Evil: A.R.G.U.S. when Steve Trevor's partner tells him about how their undercover agents have been revealed and are being killed. She says that after their undercover squad was exposed in France, they were killed by The Hyena, who is rumored to eat people whole. The fact that The Trickster hasn't shown any symptoms of becoming a were-hyena likely means The Hyena's transformation was due to the drug.
The Hyena teamed with Mindboggler, The second Enforcer, and Multiplex to battle local officers in Pittsburgh. They are quickly dispatched by a passing-by Cyborg. It has not been revealed if this is a completely different Hyena or one of the earlier Hyenas.