The son of Oni Juuma, a powerful Yakuza crime boss, Hirohito was taught the family business at a young age by his father after his mothers death. After moving to the United States, his father was captured and arrested by the Green Hornet. Before his father's death in prison, he was named head of the family.
Hirohito did not follow in his father's criminal footsteps, being too closely watched by international agencies. He instead started a video game company called Juumasan Games, quickly making a fortune off of violent video games. He also hired his siblings Katsuko and Kenji, but still only confided in the family advisor Hayoto. He began preparing to restore his family honor and achieve his vengeance against the 'killers' of his father - the Green Hornet and Kato.
Using 3-D simulations to train himself to battle them, he finally left for Japan to find Kato, whose identity was public. Entering Kato's home, Hirohito found only Kato's wife Sayomi, whom he killed quickly and painlessly. He was even able to defeat Kato in combat due to Kato's rage, but was forced to escape when the authorities arrived.
He grew obsessed with vengeance and soon had his laboratories develop him a costume and weaponry similar to the Green Hornet's. He then used the identity to kill all the other Yakuza bosses in Japan as the Black Hornet.
After a deal with Mayor Scanlon of Century City, Hirohito again took the guise of the Black Hornet to put the mayor ahead in the race for his re-election. The Black Hornet went to his campaign party and killed the original Green Hornet - Britt Reid. Through the deal, it was also revealed that Hirohito's father was in fact still alive and freed from prison. His father wished to continue with his weath and power, content at the death of their enemy. Hirohito was still not satisfied, as he knew Brit's namesake had taken over the mantle of his father.