Please note: Full spoilers for Yakuza 0 and Yakuza Kiwami ahead.
The release of Yakuza 0 in 2015 added a new depth to the Yakuza series, particularly to its first entry. Coming a year before Yakuza 1 was remade as Yakuza Kiwami (and with both being released in Europe and North America in 2017), it gives protagonist Kazuma Kiryu an origin story. More than that, though, it gives depth to the antagonist of Kiwami, his oath brother Akira Nishikiyama.
Nishiki's turn from devoted brother to power-hungry antagonist in Yakuza Kiwami has been discussed extensively, and nowhere better than in the YouTube video "Making a Villain." The YouTuber Oni traces what 0 (and some additional cutscenes added to Kiwami) bring to Nishiki's character. They demonstrate how the game shows Nishiki the mentor, the best friend, the vulnerable human being, and how these facets ultimately contribute to his downfall.
But 0 also links up with the key theme of fatherhood that underpins Kiwami (and the series as a whole). Taken together, both games demonstrate how both brothers were failed by their parental figure, Shintaro Kazama - and that this failure is the true tragedy of the arc.
Clearly, this theme comes from a specifically Japanese perspective, and is tied to the structure of yakuza families, something which deserves its own essay that I wouldn't be qualified to write on. But worldwide, people are devoted to their fathers, and the brothers' commitment is made clear - especially Kiryu's. They, along with tragically underwritten love interest Yumi Sawamura and Nishiki's even more tragically invisible younger sister, were raised in an orphanage run by Kazama, a high ranking member of the family. "You and I were just a couple of orphans, and he took us in," Kiryu says to Nishiki. "I'd do anything to repay him, but all I can give is my life."
Kazama is absent throughout Yakuza 0, having been jailed. Yet it slowly becomes apparent that he has a hand in everything that has been happening as he attempts to retain and grow his power within the family. Not only do his decisions continually place Kiryu in additional danger, he completely declines to communicate directly with him, leaving him in the dark about much throughout the game.
Kazama's actions cause many other characters to question Kiryu's devotion to his father figure. Lieutenant Daisaku Kuze calls Kazama a "cruel son of a bitch." He takes Kiryu's earlier interpretation of his childhood and twisting it, using almost the same words to paint a very different picture. "He takes in these kids and then he owns them for life...you'd be willing to give your life for him, wouldn't you? That's some brainwashing kind of shit right there."
Kuze's view is easy to disregard, particularly within Yakuza 0. As players, we see the world from Kiryu's perspective, while Kuze is an antagonist. But Kiwami's stronger focus on fatherhood as a theme challenges that assumption.
Kiwami begins with Nishiki killing patriarch Dojima in defense of Yumi. Kazama refuses to get involved, and Kiryu ends up taking the fall for his brother. While he spends 10 years in jail, Nishiki struggles to find his place without Kiryu. Yumi goes missing, and his sister dies of a terminal illness. This loss of his entire family unfortunately loses some of its emotional intensity given the lack of set-up. Even with Yakuza 0 giving backstory, Nishiki's sister never appears and Yumi has only a brief scene in Kiwami.
But still, the key is that Nishiki is alone. Like in 0, Kazama appears absent, but unlike Kiryu in 0, Nishiki doesn't have a support network of many other characters. After constantly making it clear that Kiryu is the favoured son, Kazama has only one interaction with Nishiki in the game: a public rebuke. In earlier flashbacks, Nishiki only gets to speak to his intermediary. And where Kazama attempted to give Kiryu an out of the dangerous yakuza life in 0, there's no indication he ever did the same for Nishiki.
There's a whole chapter in Yakuza Kiwami called Father and Child. It contains subplots about three families (though consistent with the game's failure in its portrayal of women, there are no mothers involved). Two of them are combined: an information broker's son wants to run away with the daughter of a yakuza patriarch, and Kiryu must step in to protect them. The third sees Kiryu's detective friend reconnect with his own daughter. All of them are somewhat unrelated to the main story, and would feel filler-ish were it not for their clear connection to the themes of the game.
All three of the fathers centred in this chapter are shown to have contributed to their children's struggle through their absence. Though each character does so in different ways, all of them are ultimately held accountable for their failures, mostly caused by their absence. It's directly comparable to Kazama's actions towards Kiryu in 0, and Nishiki in Kiwami.
Each chapter in Kiwami ends with a flashback to Nishikiyama's struggles during Kiryu's jail time. Father and Child ends with him being confronted by Kazama's rival Futoshi Shimano. "Who knew Kazama had such a cruel streak," he says, echoing Kuze's sentiments from 0. "If he really wanted to help ya come up, why did he give ya men ya couldn't control?" Like Kuze, Shimano is an antagonist. There's no explicit confirmation that what he says is true. But this dialogue's placement at the end of this chapter is no accident. It squarely places Kazama in the same position as the absent and undeserving fathers demonstrated earlier in the chapter.
When Kiryu is released from prison, Nishiki ought to no longer be alone. It's true that by this point he is pretty far gone, having murdered at least three people, but Kiryu initially refuses to give up on his brother. It's only after he discovers Nishiki attacked Kazama that the pair are shattered. Compare Kiryu's immediate reaction of punching Nishiki here to his instant forgiveness of Kazama's deathbed confession that his orphanage cares for the children of parents he killed as a yakuza hitman. His admission to killing Kiryu's own parents, along with Nishiki's, Yumi's, and countless others rolls right off Kiryu, who doubles down on saying Kazama was his true father. (As players, though, we might begin to reconsider Kuze and Shimano's perspectives.)
The brothers cannot come back from this point. Of course the final fight between them is enforced by genre convention, but it's also a natural conclusion of the arc of these two characters as Kazama's sons. His favouritism sowed discord between them, his absent parenting left them struggling with things far beyond their control, and their whole world taught them no form of conflict resolution beyond violence.
As well as fatherhood, Kiwami deals deeply with themes of accountability. Kiryu eventually realises that taking the fall for Nishiki prevented both of them from dealing with the full weight of their responsibilities. Nishiki's final act is to save Kiryu and his adoptive daughter Haruka and destroy the money he had once been fighting for. As Oni puts it in their video, "the ultimate apology, the only way Nishiki knows how: totally over the top and driven purely by emotion." Kazama's final action is also to save Haruka; another apology, this time symbolic of the hope that Kiryu won't repeat his mistakes as a father.