He only knew his father for the first 6 years of his life before lung cancer took him. After a few years, his mother, Haruna, a fashion designer and seamstress, adopted the half-crazed Megumi . It took a while to warm up to her, but when he saw how hard Megumi fought against Hiro in the courtroom, he took her side, unconditionally.
His favorite franchise is Kamen Rider, a series that revolves around a masked motorcycle rider fighting monsters. He’s studied it so extensively; he calls himself a Rider Encyclopedia.
He moved to America with his mother and adopted older sister. Sadly, he had some racial slurs directed at him and his family. That inspired him to take up the art of the sword. After a joke from Megumi, where she said that he acted princely, trying to satisfy honor, Hiroki joked that if he was a prince, then she was a princess that needed his defense.
Haruna said that they were practically Feudal Nerds, so Megumi took that idea and Hiroki helped her found the Feudal Nerd Society, a club where you could dress fancy if you wanted to. Haruna was a part of it as well as she designed most of the outfits each member wears.
4 years later, Hiroki, along with the rest of his friends, obtained a Vortex Driver and fulfilled his dream of becoming a Kamen Rider called Sengoku and fighting alongside Takeshi Hongo, the first Kamen Rider.