Kyojuro Rengoku was the first Hashira in Demon Slayer: Kimetsu no Yaiba, who won the hearts of everyone by showcasing his capabilities in the Mugen Train Arc. His fight with the Upper Rank 3 Akaza became the most enthralling action sequence in the history of Demon Slayer. Apparently, he is said to be the most formidable Flame Hashira in the corps.
Although Kyojuro walked in the footsteps of his father, Shinjuro Rengoku, he never really got to learn anything from him. Kyojuro learned everything from the guidebook of Flame Breathing. Although Shinjuro is regarded as one of the strongest Pillars in the Demon Slayer Corps, it is still unbelievable that someone like him would voluntarily quit being who he is.
What is the actual reason for Shinjuro to abandon his position as a Hashira in Demon Slayer?
Shinjuro is one of the strongest demon slayers in history. Due to his dominant Breathing Technique, he is considered to be an exceptional swordsman in his days of being a Hashira. However, one fated day after the death of his beloved wife, Ruka Rengoku, Shinjuro detached himself from the world he knew, forsaking the vow he took as a demon slayer.
Ruka was healthy and fine, but after giving birth to Senjuro (Kyojuro's younger brother), she developed a terminal disease that left her bedridden and waiting to die. After the loss of his wife, Shinjuro cultivated hatred as well as agony and started drinking while doing his duties.
Kyojuro remembers that his father was not always a contemptuous person, as he was praised for his innate skills and valued his duty more than anything. Shinjuro was the person who swore to protect others from demons and he was determined enough to give away his life in the process if it came to that point.
However, Shinjuro gave up being a Hashira, and before his retirement, he tore apart the Generational Flame Hashira Record, which had been passed down from generation to generation in the Rengoku family.
Shinjuro's stone-heartedness blinded him to the extent that even the death of his elder son Kyojuro didn’t bother him. Moreover, he started berating his son and his capabilities in front of Tanjiro, and both of them got infuriated and fought each other. The fight ended up with Tanjiro’s signature headbutt, which made Shinjuro fall to the ground and left the former unconscious.
Deep down somewhere, Shinjuro really cares about both his sons. As seen in the first episode of the Entertainment District arc, where after receiving the final words of Kyojuro, he burst into tears. Although Shinjuro was never kind to his son, he really had high hopes for him.
Sometime later, Shinjuro apologized to Tanjiro through a letter and explained all the events. He showed a great sense of pride for Kyojuro, who trained consistently and learned the Flame Breathing just by reading 3 volumes of it. Shinjuro also brought an exceptional candidate like Obanai Iguro, who became a Pillar in the Demon Slayer Corps.
When the Snake Demon was about to kill Obanai, Shinjuro came to the rescue and defeated the demon with utmost efficiency. Shinjuro even tried to reunite Obanai with his cousin. However, after looking at the cousin’s attitude, Shinjuro resorted to taking Obanai along with him.