Senjuro has a similar appearance to his brother, Kyojuro Rengoku, and father, Shinjuro Rengoku, possessing unruly golden hair that flares out around his face, becoming red near the ends above his ears and shoulders, that he wears messily tied back behind his head. He has fair skin and large, gold eyes that redden near the pupils, that are slanted inwards and shaped so as to give him a determined yet kind appearance.
Senjuro is originally seen to be very sad and rattled by his brother's death and his father's verbal abuse, but, after spending some time talking to Tanjiro Kamado, his true personality as a kind, caring individual who can usually see the best in a situation or person shines through. He has hope for his future, even though he knows, by not becoming a Demon Slayer, he has broken his family's ancient tradition, because he knows that his older brother, Kyojuro, would forgive him for leaving such a scar on their history.
It's noted that, at some point in his life, Senjuro was to become Kyojuro's successor and so had been training to become a Demon Slayer. However, due to his lack of skill in the field, Senjuro gave up on that dream and decided to spend his life helping people in other ways.
Senjuro first appears in Kyojuro's dream, asking if their father was happy to hear the news of Kyojuro's inauguration as the Flame Hashira. He is met with a different response and is saddened to hear the hopelessness of their father's mental situation.
Kyojuro later gives him words of encouragement and reassures him that things were not as bad as they seemed and to do his best throughout life.
Senjuro appears once again after the battle with Enmu, Lower One of the Twelve Kizuki, and Akaza, Upper Rank Three. He is seen sweeping the entrance outside of his home, wearing a saddened expression, as a worn out and injured Tanjiro arrives at his house in the hope of delivering Kyojuro's final message to his family. Tanjiro calls out to Senjuro and explains why he is there, but Senjuro just looks at him, confused, saying he already knew what had happened to his brother and asking the Demon Slayer if he is alright.
Suddenly, Senjuro's father, Shinjuro, appears from his house, yelling that what Tanjiro's come to tell them is useless and belittling and cursing Kyojuro for becoming a Demon Slayer since he always knew it would just get him killed. Tanjiro and Senjuro are shocked by his words, but remain silent as he continues to rant about how people's powers are already decided for them at birth, but Kyojuro still went and became a Demon Slayer despite the fact that he had none. At the sound of these sharp words, tears begin to form in Senjuro's eyes, but, instead of stopping and comforting his son, Shinjuro tells him to stop making that pitiful expression since the funeral is already over. Angered by this display of cruelty, Tanjiro asks Shinjuro to stop talking to and about his sons in that way, which only causes the latter to turn his attention to him and yell at him to get off his property, and, defiantly, Tanjiro says that he is a Demon Slayer.
The container Shinjuro is holding smashes to the ground as he considers what this could mean, and, jumping to the first conclusion that comes into his mind, he angrily asks Tanjiro if he is a Sun Breathing user. Without waiting for an answer, Shinjuro swiftly attacks Tanjiro and pins him to the ground, forcefully slapping Senjuro away when he tries to pull his father off the injured Demon Slayer. While Shinjuro is distracted by his son, Tanjiro swiftly delivers a blow to his face, forcing him off of him, and, as Tanjiro's patience has finally worn thin, furiously asking him what the hell he's been doing, badmouthing Kyojuro, who gave up his life, and then hitting the only son he has left. Shinjuro shouts that he knows Tanjiro is just mocking him because he's a Sun Breathing user who's just full of himself because he can use the original Breathing Style, and Tanjiro shouts back that he can never get full of himself because of how badly he's just been beaten down. Running at Shinjuro, the young Demon Slayer tells him never to speak ill of Kyojuro again.
Senjuro tries to warn him that Shinjuro is a former Hashira, but, because he's being driven by so much anger and sorrow, Tanjiro continues to charge at him. Shinjuro hits Tanjiro's face, ignoring the yells of protest from his son, and begins to repeatedly land blows on Tanjiro's body until the young Demon Slayer hits him square in the jaw with a spiralling headbutt, causing them both to fall down on top of each other.
Sometime later, inside the Rengoku estate, Senjuro brings Tanjiro some tea, and Tanjiro thanks him for his kindness. He also apologizes for knocking Shinjuro out with his headbutt and asks if he's alright, to which Senjuro replies that he thinks he'll be fine. Senjuro says that he feels relieved, as Tanjiro was able to stand up to his father as he insulted Kyojuro when Senjuro couldn't say a word. He wonders aloud how his brother's last moments played out, and Tanjiro explains that he went out gloriously. Tanjiro apologizes again to him for not being of any help to his brother, but Senjuro tells him not to worry about it, and that that response would've been what Kyojuro would say.
Senjuro then goes off to look for the book Tanjiro requested to have a look at, and returns with a book entitled "The Generational Flame Hashira Record," which Tanjiro takes from him, thanking him. They sit down to open the book, but only discover that it's been torn to shreds. Tanjiro asks Senjuro if it's always been in this condition to which the latter replies that it definitely hasn't because it was supposed to have been preserved very well. He guesses that it was his father who tore it apart, and apologizes to Tanjiro, who tells him not to worry since it wasn't his fault.
Senjuro feels bad that he came all this way only to discover that the book he'd wanted had been ruined, but Tanjiro once again tells him that it's fine since he can always try and discover more about the technique through training himself rather than reading. He explains that his body becomes a lot less mobile when he uses his fully-focused breathing, so he needs to increase his stamina in order for him to perfect the technique. Because of the regret he feels due to his inability to save Kyojuro, Tanjiro says that he will keep moving forward and strive to become a great Hashira one day, just like Kyojuro had been. Senjuro is touched by this, and tears up, smiling to himself. He tells Tanjiro that his brother never had a successor, and that he didn't fill the role because he was too weak for his Nichirin Blade to even change color. However, he explains, he can still do other things that are useful to people, and, even though he knows breaking the succession of Flame Hashira, he knows Kyojuro would forgive him. Tanjiro encourages him, telling him to go down the path he thinks is right and saying that he will headbutt anyone who badmouths him, to which Senjuro replies that it would probably be best if he stopped doing that.
As Tanjiro prepares to leave, Senjuro says that he will try and restore the Flame Hashira book for him by using other sources and asking his father about its contents, saying that if he figures anything out he'll send a crow over to Tanjiro right away. He thanks Tanjiro for their talk, and offers him the gift of his brother's Nichirin Blade guard, which, after some convincing, Tanjiro accepts. With that, he wishes the young Demon Slayer well and waves him goodbye.
Senjuro turns and heads back into his house, finding his father sitting alone and attempting to talk to him about what had happened earlier, but Shinjuro just tells him to be quiet and leave him alone. With no other choice, Senjuro exits Shinjuro's room, leaving him only with the message that was left for him: "please take care of yourself."
Abilities and Powers
Despite coming from a long line of Flame Hashira, Senjuro never possessed any great skill in the art of Demon Slaying. As hard as he tried, Senjuro couldn't acquire the degree of swordsmanship needed to make his Nichirin Blade change color, and eventually gave up on following in his brother's footsteps.
- Senjuro exchanges letters with Tanjiro.
- Senjuro loves reading Tanjiro's stories about Zenitsu and Inosuke. For him, they are so entertaining that he can't stop laughing.
- In the Kimetsu Academy, Senjuro is a first year student in middle school. Average in everything.
- Senjuro loves to read.