The first five steps of the Hero's Journey. They include: the call to adventure, refusal to the call, the supernatural aid, crossing the first threshold, and the belly of the whale.
Departure
The first step of the hero's journey is the Call to adventure. Usually the hero wants to do something more with their life but does not openly state it. Kazuma's call to adventure begins at the beginning of Yakuza when he and Shinji got to collect a debt. For all intent and purpose it is a tutorial battle, however we see that Kazuma is quite disinterested in the idea, refusign to talk to Shinji about it too much. Instead he merely does his job as required. He just wants to get it over with and go see his friends.
The next step is the refusal to the call, often it is due to the hero's humility. In fact, Kazuma doesn't like to fight and he's only doing the job because he is good at it. Instead of going straight back to work and doing other jobs, he heads to Serena to hang out with his friends. When he is with his them, he seems more comfortable. He would prefer to always be with them, but cannot due to work and the obligations he must fulfill.
Kazuma's supernatural aid is his inhuman strength. He is feared and revered as the physically strongest man in Kamurocho and is even considered Legendary. His abilities as a fighter set him so far above everyone else that people have been known to run from him or try to kill him (in some cases on more than one occassion). His ability to fight is what gets him through the successive games and causes him trouble, it is also what pushes him into the deepest depths of the Yakuza Underworld.
The first threshold is crossed when Kazuma gets a call from a frantic Nishiki. With Yumi being taken Kazuma stands up and rushes to their aid. All the confidence he needs is the ability to protect his friends. He heads into his journey to save his friends, nothing more and nothing less.
His life gets turned upside down and he departs from the only life he knew into the belly of the whale. Kazuma's willingness to jump to Yumi and Nishiki's aid takes him straight to the belly of the whale where he faces a real challenge - save his friends and betray the family or betray his friends who were his first family. The head of his family lays dead before his best friend, the women he loves cowers in the corner. He takes the gun from Nishiki and tells him to look after Yumi. Kazuma takes the fall for shooting Dojima. He cannot allow Nishiki to go to prison because Nishiki's sister is sick. And he feels too strongly that he could not protect or care for her or Yumi. He entrusts Nishiski to the task and ends up in prison.