Shinjuro strongly resembles his son, Kyojuro. Their facial structure, eyebrows, and hair are almost identical.
Shinjuro wears a traditional kimono robe. In the Dimensional Infinity Fortress Arc, he is shown with a katana with a flame guard similar to Kyojuro's and what appears to be a flame pattern on the sheath.
Shinjuro is a very short-tempered man, especially regarding the Demon Slaying 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 Dimensional Infinity Fortress alongside the other Demon Slayers and vows to follow Kyojuro's example and not bring shame to the Rengoku family.
Sometime before the Demon Train Arc, Shinjuro tore apart the Generational Flame Pillar Record of the Rengoku family.
Following Kyojurou's death after his battle against Upper Moon Three Akaza, Tanjiro Kamado visits the Rengoku residence in order to learn more about his Dance of the Fire God, where he mets Shinjuro and Senjuro. Upon hearing about Kyojuro, Shinjuro immediately begins berating Tanjiro, insulting Kyojuro in the process, causing Tanjiro to retaliate in anger. Shinjuro then attacks Tanjiro after recognizing Tanjiro's earrings as those of a user of the Breath of the Sun, beating up the injured Tanjiro until he is eventually knocked out by a headbutt.
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 tells Shinjuro 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 Moon Four Nakime's summoning of the Dimensional Infinity Fortress, Shinjuro is seen sitting in the fortress, ready to fight, alongside former Sound Pillar Tengen Uzui. He praises Kiriya Ubuyashiki's ability to command the Demon Slaying Corps even after his father's death, and silently vows to follow Kyojurou'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 Pillar of the Demon Slaying Corps, it can be assumed that Shinjuro is an extremely powerful swordsman. In a brief fight, he was able to completely overwhelm Tanjiro Kamado 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".