Shinjuro strongly resembles his son, Kyojuro Rengoku, possessing unruly orange hair that flares out around his face, becoming redder near the ends above his ears, that he wears messily tied back behind his head. He has fair skin, bushy black eyebrows and angry, orange eyes that redden near the pupils, that are slanted inwards and shaped so as to give him a furious appearance.
Shinjuro wears a traditional kimono robe. In the Infinity Castle Arc, he is shown with a katana with a flame guard similar to Kyojuro's, with what appears to be a flame pattern on the sheath.
Shinjuro is a very short-tempered man, especially when regarding the Demon Slayer Corps and his son, Kyojuro, with the mention of either causing him to grow furious. He appears to be abusive, harshly insulting the recently deceased Kyojuro and knocking Senjuro to the ground during his fight with Tanjiro. He is also an alcoholic - his first action upon awaking after being knocked out by Tanjiro is to buy alcohol, and he is often seen drinking.
However, it is later revealed that Shinjuro truly cared for Kyojuro despite what he said to Tanjiro, and the memory of Kyojuro causes Shinjuro to begin breaking down in tears.
He later enters the Infinity Castle alongside the other Demon Slayers and vows to follow Kyojuro's example and not to bring shame to the Rengoku family.
Sometime before the Demon Train Arc, Shinjuro tore apart the Generational Flame Hashira Record of the Rengoku family.
In Kyojuro's Blood Demon Art-induced dream aboard the Demon train, Shinjuro first appears, lying on the floor of their house and gazing out of the open doors at the garden. Kyojuro remembers that he's supposed to have just reported to him that he has become a Hashira and his father, unimpressed, says that it was pointless because neither himself nor Kyojuro can ever become great people.
Kyojuro recalls that Shinjuro wasn't always this dismissive and that he used to be a man of great skill and passion for Demon slaying, but suddenly, one day, the man, who had raised himself and Senjuro Rengoku so passionately, simply gave up on the blade.
Shinjuro appears once again after the battle with Enmu, Lower Rank 1 of the Twelve Kizuki, and Akaza, Upper Rank 3. He suddenly 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.
After Tanjiro leaves, Senjuro returns to the house and finds Shinjuro drinking on the porch. Upon hearing Senjuro's voice, Shinjuro immediately lashes out and shouts for him to leave; however, before Senjuro leaves, he says Kyojuro's last message: "please take care of your body." Shinjuro then puts down his drink and breaks down in tears over Kyojuro.
Following Oyakata's death and Upper Rank 4 Nakime's summoning of the Infinity Castle, Shinjuro is seen sitting in the fortress, ready to fight, alongside former Sound Hashira Tengen Uzui. He praises Kiriya Ubuyashiki's ability to command the Demon Slayer Corps even after his father's death, and silently vows to follow Kyojuro's example and risk his life for their protection and not bring shame to the Rengoku family.
Abilities and Powers
Due to his status as the former Flame Hashira of the Demon Slayer Corps, it can be assumed that Shinjuro is an extremely powerful swordsman. In a brief fight, he was able to completely overwhelm Tanjiro in hand-to-hand, but was eventually knocked out with a powerful headbutt. He was noted by Tanjiro to not "move like an average person", hinting to his above average physical abilities as a trained/seasoned demon slayer.
- First Form: Unknowing Fire (
壱ノ 型 不知火Ichi no kata: Shiranui?) - The swordsman charges towards his opponent at high speed and decapitates them in one or multiple slashes.
- Second Form: Rising Scorching Sun (
弐ノ 型 昇り 炎 天Ni no kata: Nobori en ten?) - The swordsman uses an upwards moving slash attack.
- Fourth Form: Blooming Flame Undulation (
肆ノ 型 盛 炎のうねり Shi no kata: Sei en no uneri?) - The swordsman spins his sword in a circular motion, deflecting his enemies attacks.
- Fifth Form: Flame Tiger (
伍ノ 型 炎 虎Go no kata: Enko?) - A series on sword slashes that take on the form of a tiger.
- Ninth Form: Rengoku (
玖ノ 型 煉 獄Ku no kata: Rengoku?) - A devastating dashing slash initiated from a high stance. The technique carves a deep impression in the ground.
- It is unknown why, but Shinjuro hates animals.
- (In response to Tengen Uzui) "...Right. These kids at a very young age are encouraging me so much. Just as Kyojuro did, I will risk my life and protect them. So I will not cast shame on the Rengoku family name."