Renzo is the son of Kawakami Araya, a member of the Ittou-ryu. Among the Ittou-ryu members who killed Rin's father, Araya raped Rin's mother, O-Toki, before spiriting her away. Anotsu Kagehisa, who was present, put in defense at Rin being raped, but allowed the assault of her mother.
Renzo and Rin cross paths during a festival where Renzo, being younger and more brash than Rin, annoys her and then finds himself in trouble. Rin steps in to save Renzo's life, and he then invites her to his home, as his father told him to always invite kind people over for tea.
When Araya returns home, he and Rin recognize each other while Renzo is sent out on an errand, and a fight ensues. Manji arrives in time to protect Rin, and eventually kills Araya just as Renzo returns home. Not knowing anything about the circumstances of his father, nor even the name "Ittou-Ryu", Renzo takes up his sword to kill Manji. Rin then helps fabricate Manji's death by using Manji's severed arm in a crude grave to "prove" he is dead. Every time Rin and Renzo cross paths, Rin's only goal is to protect him from the truth of his father and her family.
Renzo returns to the story just as Rin is helping to break Manji out of prison. Finding Manji alive, Renzo steals Manji's newly-severed arm and runs away with Shira, no longer trusting Rin and growing bitter over the realization that he had been tricked, and that his father's killer was still alive and well.
It is revealed that Renzo and Shira shared a prison cell while Shira was being experimented on during Manji's time in prison. Renzo was abused and beaten by Shira, and Renzo became his literal right-hand man. After Shira and Renzo escape from prison, Shira eventually locates Manji and Rin and prepares for the ultimate fight with Manji. To help make Manji angry, Shira first kidnaps Rin and ties her, semi-clothed and half-suspended in a freezing pond where she will slowly freeze to death. Shortly after Manji runs into Shira, Renzo returns to the dock and talks to Rin. He does not help her, but he is obviously conflicted, remembering how she saved his life. He cannot entirely forgive her for lying to him, however.
After Manji and Rin are saved, and Manji and Magatsu finish off Shira, Renzo goes to see Shira just before he is eaten by wild dogs and then returns to the hut where Manji and Rin are recovering with Tanpopo, Meguro, and Magatsu. He takes up his sword again, planning to cut off Manji's arms, frustrated at not understanding why everyone always saves Manji and no one saved Shira. Rin comes to Manji's defense, saying she hired Manji to kill Renzo's father, and offers up her own arms for Renzo's retribution, though she still refuses to say anything about why she did so to continue protecting Renzo from the truth.
Magatsu finally tells Renzo about his father, much to Renzo's shock, as Magatsu was one of the Ittou-Ryu members, along with Araya, who broke into Rin's family home and killed her father. Renzo tells Rin he always assumed something bad happened between her and his father.
In the end, Renzo lives with and studies painting under Master Sori. He refuses to see Rin, though not, he says, because he blames her for his father's death, but because he thinks she's a fool for her entire revenge quest.
Araya: Renzo's relationship with his father, Araya, was very ordinary and loving. Renzo never had any clue his father was a swordsman at all, only assuming Araya was an artist, making masks to sell at festivals. Renzo takes the murder of his father by Manji very hard, and decides to start his own path of revenge against Manji. He hates Shira and knows he's a bad person, but is also young and hurt enough to be manipulated by him.
Rin: Renzo's relationship with Rin is complex because they start off as tentative friends and end up confused enemies. After finding out Rin lied to him about Manji's death, and having been under Shira's thumb in prison, Renzo turns from Rin and re-starts his plan to see Manji killed.
When Rin is kidnapped by Shira and put in a life-threatening situation, Renzo is torn to the point of crying, but does not help her. When Tanpopo rescues Rin, Renzo does help them, and Tanpopo has a short conversation with Renzo about deciding how he feels about Rin.
When Rin tries to protect Manji from Renzo's sword, Renzo still goes after Manji, being sure to avoid Rin in the process. He never actively hurts or sabotages Rin, even though he does really want to kill Manji. Rin, meanwhile, never explains her history with Renzo's father, wanting him protected from the truth.
Shira: Renzo and Shira meet while in prison. Shira rapes him and a female inmate, but the female dies. Since Renzo survived his brutality, he makes the boy be "his hands", telling him what to do, helping him eat, and satisfying him sexually. Over time, Renzo becomes dependent on him.
Under Shira's brutal influence, and still broken-up about his father's death, Renzo is particularly angry and betrayed when he finds out Rin lied to him about Manji's death. His and Shira's shared disgust for Manji leaves Renzo in a vulnerable position where Rin is concerned. When Shira is left for dead, Renzo sits by him, torn up about losing him and being alone again. Shira tells him he's better off staying away from people like him.