Kokujin and Hiroki's relationship is one of pure hatred, Kokujin having the upper hand on Hiroki by simply being stronger than him physically.
Kokujin slept with Hiroki's gf who's name is Nao, she was the first one to betray Hiroki for Kokujin...
But he didnt stop there, Kokujin also slept with Hiroki's mom and sister, to assert his dominance over the situation...
After Hiroki trained for months and months everyday to finally beat up Kokujin, the result is on fate's hand, help Hiroki!