Demon Slayer's Hashira are peculiar individuals with overwhelming power, skill, and presence. But even among this group of eccentric people, Kyojuro Rengoku still manages to stand out from his peers due to his upright and extremely bright personality. Though to be fair, his fiery hair is also one of the biggest reasons why he sticks out.
That being said, most Hashira chose to be a Demon Slayer because they too were once a victim of a vicious demon attack. After all, most of them had lost their loved ones to a raging demon. That exact thing happened to the Insect Hashira, Stone Hashira, Wind Hashira, and also Water Hashira. That extreme sadness and anger are the fuel and motivation behind their unparalleled strength. But that is not the case at all for Rengoku. He didn't lose any family members to demons. So why did he decide to become the Flame Hashira?
To Continue The Tradition
Before Kyojuro took over, his father, Shinjuro Rengoku, was the Flame Hashira for quite a long time. It is not only limited to this father and son pair, seeing as for generations, the Flame Hashira always came from Rengoku lineage.
The first head of the Rengoku family learned the Sun Breathing technique from the first demon slayer, Yoriichi Tsugikuni, and then adapted techniques into the current Flame Breathing that would be passed down from one generation to the next.
As the eldest son of the current Rengoku family, it falls upon Kyojuro to carry on their traditions. Not to mention his brother, Senjuro, doesn't have the physical ability needed to properly use Flame Breathing. So Kyojuro basically has little to no choice but to dedicate himself to the way of the flaming swords, and become the next Flame Hashira.
To Fulfill The Promise to His Mother
Another big reason why Kyojuro is so determined on becoming the Flame Hashira was because of his promise to his late mother, Ruka Rengoku. Ruka was a woman who had a very strong sense of justice. She firmly believes that it is the responsibility of the strong to protect those who can't protect themselves. That was the value and way of life that she wanted to teach to her sons.
Unfortunately, Ruka got infected by a terminal disease and passed away before Kyojuro managed to become a Hashira. But even on her deathbed, Ruka still reminded her son to uphold that noble principle. At that time, Kyojuro promised his mother that he would do everything in his power so that he could help as many people as he could. That promise to his mother is the one thing that keeps the fire burning in his heart no matter how hard the situation might be.
To Prove Himself to His Father
As mentioned earlier, Shinjuro Rengoku, Kyojuro's father, served as the Flame Hashira for quite a long time. At the time of his retirement, he was the oldest Hashira among his peers. Unfortunately, the cause of his retirement was not due to injury or old age, but rather because of a broken heart.
When his wife passed away, Shinjuro was so overcome with sadness that he drowned himself in alcohol. He began to neglect his responsibility as a Hashira and also as a father. He would often yell at Kyojuro and Senjuro and told them that they have no talent as swordsmen and would never master the Flame Breathing techniques.
Kyojuro took all of that yelling and beating and turned it into motivation to get better. Despite what his father said, Kyojuro was determined to train and work hard so that he could prove his own worth to his father. That is the reason why during a Hashira meeting that his father didn’t attend, Kyojuro decided to talk to Ubuyashiki and the other Hashira and ask to be appointed as the new Flame Hashira, replacing his father who is currently incapable of fulfilling his duty. Becoming the new Flame Hashira is his way of proving to his father that he has become a capable swordsman.
To Make His Brother Proud
Kyojuro was acutely aware for a long time that his brother Senjuro was not blessed with his great physique. However, he would never belittle his brother and would always motivate him instead. Kyojuro strongly believed that hard work won't ever let anybody down.
Since his father was incapable of fulfilling his responsibility as a proper parent after their mother passed away, Kyojuro took it upon himself to be the best role model for his brother. He does his best at everything because he wants to be a good example for Senjuro and make him proud. Being a Hashira is one of the best ways to do that.
His Unwavering Sense of Justice
Thanks to the principle instilled to Kyojuro by his late mother since he was little, he grows up to be someone who has a very exceptional sense of justice. He would always prioritize saving the civilians first over defeating the enemy, which is exactly what he did during the Mugen Train arc.
Moreover, Kyojuro was never trapped into the notion that every demon is bad and therefore must be killed. He was the kind of man who judges others based on what they do, which is why he acknowledges Nezuko after knowing what she does to help other people.
All of those things combined and we got the reasons why Kyojuro decided to devote everything he has in order to become a Hashira. He may not have lots of screen time on the original series, but his overwhelming presence left such a big impact on everybody that he's met throughout his life. And that is why he deserves to take on the mantle as one of the strongest and most influential swordsmen in the world of Demon Slayer.