Kiryu Kazuma
Kiryu Kazuma is Ryuji's rival. There is something so similar about them in both strength and personality that I often just considered Ryuji a Kazuma clone. They have their differences in opinion, but when it comes down to it, both men would do anything for those they loved, though you do not see that side of Ryuji very often. Kazuma and Ryuji start out at odds with each other. Kazuma wants to bring peace to Tojo and Omi, while Ryuji wants to bring everything around him down. In fact, they meet in the Cabaret and when Kazuma gives Ryuji his name the Kansai man is "relieved" to hear he's not the Dragon of Dojima.
Kazuma does not realize that Ryuji knows who he is until Ryuji states he could "smell the blood" almost instantly. Ryuji then gives Kazuma a warning that there will be fireworks in Kamurocho. Ryuji knows who Kazuma is and revels in his knowledge, playing the fool until he feels he safely has the upper hand. Ryuji is very careful and actually fairly hospitable while Kazuma is intervening on behalf of his men. Ryuji apologizes and tries to treat him to a drink. Kazuma declines and they talk, but Ryuji is ever watchful and very careful of his choice of words until their parting. He offers a small smirk with his warning and leaves.
Although Ryuji is combative and on occassion rude, he does have some respect for his rival. When Kazuma is injured after going to reclaim Haruka from Sengoku, Ryuji steps in and saves her. He kills Sengoku and throws Haruka over his shoulder, takes her to Kazuma and tells his rival that he still has some time left on the grace period. Ryuji could have killed him then. He didn't. Why? He wanted Kazuma at his strongest. This could be attributed to when they first fought and Kazuma beat him, but stated it did not count becuase Ryuji had previoulsy fought Daigo.
Kazuma is the strongest man in Kamurocho, he is legendary. Ryuji feels that he cannot be called the Dragon of Kansai until he and Kazuma fight. One of them has to win to claim the title of dragon. Ryuji loses at the end of Yakuza 2. In Yakuza Dead Souls, Ryuji discards the dragon title and insists that people do not address him as such. He hates the title and uses it only to deliver messages to Tojo (knowing they would instantly let him in) and to help Kaori (as Go-Ryu clan was on friendly terms with the Ryurei who were extorting her brother). Like Kazuma, he would not willingly throw around the Dragon title unless it somehow benefited someone else.
Partner of the End
In all actuality, Kazuma represents the strength that Ryuji cannot achieve. Ryuji will never be as strong as Kazuma. As Ryuji gets older he realizes this and seems to harbor no ill feelings towards the man. In Yakuza Dead Souls, Ryuji and Kazuma are reunited but not as enemies. Ryuji wants to stop Nikaido. Kazuma wants to find Nikaido. They share a goal and head to Millennium Tower together. They scale the building and bring down Nikaido and DD's Sweet Death.
I find it interesting that Ryuji is short with Kazuma. He tells the other dragon to "pull his weight" and to "hurry his ass up." But he's not necesssarily rude about it. It seems more likely that he is teasing the man he could not beat, though he knows he can trust him. In part, I believe that Ryuji knew if he couldn't finish the work - Kazuma could. In fact, when Nikaido morphs and brings down part of the Millennium Tower Kazuma is not fast enough to protect Haruka. Ryuji saves her from being crushed but with all the debris is unable to finish what he started. He entrusts to to Kazuma as a "fair trade."
When Kazuma stops him from shooting DD, he becomes greatly annoyed showing he still does not see eye to eye with Kazuma. Ryuji will work with him as long as they are working towards the same goal, but he does not have to agree with him. When the helicopter crashes into the ground after being pulled down by zombies, Ryuji comments "Yea, that's about what he deserves." He seems content to know that everything has stopped and that things will return to normal again. Haruka and Kazuma visit him on their way through Kamurocho at the end where he treats Haruka to some Takoyaki.