Here's the backstory of Demon Slayer fan-favorite Kyojuro Rengoku explained. The Demon Slayer manga was first published in 2016, while the anime quickly followed in 2019. It's already one of the most popular anime franchises of recent times, while the movie spinoff became Japan's highest-grossing film ever in 2020. The series follows Tanjiro, who joins with the Demon Slayer Corps in the hopes of finding a cure for his infected sister Nezuko. The duo travel around fighting hideous - and quite creative - monsters of every variety.
Demon Slayer has connected with viewers because of its beautiful artwork, great characters and empathy of its main hero. Even against the odds, Tanjiro always tries to do the right thing and hold on to his morality. While most of his fellow slayers only see the demons as monsters, Tanjiro takes pity on them feels no pleasure in their defeats. Towards the end of Demon Slayer's season 1, Tanjiro gets to meet all of the Hashira, who are the most powerful warriors in the Demon Slayer Corps. This includes Kyojuro Rengoku, the Flame Hashira.
Renjoku is instantly recognizable due to his flame-red hair and upbeat personality, but he, Tanjiro and Nezuko don't get off to a good start, as he and the Hashira want to kill the latter because she's a demon. After this misunderstanding is cleared up, Rengoku leaves for a mission, which sets up the role he'll play in Demon Slayer The Movie: Mugen Train. Both the film and manga give insight into his childhood, and how he became the slayer so well-liked by the fanbase.
It's revealed Rengoku's mother Ruka died when he was young, and she's the one who pressed upon him that he must use his immense strength and abilities to protect the weak. This is one of his core principles in Demon Slayer, but while he was close to his father - the former Flame Hashira Shinjuro - this changed when Ruka died. Shinjuro fell into a depression and didn't encourage his son's success, though Rengoku loved him all the same. He also had a younger brother named Senjuro, who succeeded Rengoku as Flame Hashira following the events of the movie.
Rengoku is also known for talking in a loud, energetic manner, which was revealed in a manga spinoff to be partly down to the slayer rupturing his own eardrums to defeat a flute demon. Despite a somewhat tragic backstory, one of Rengoku's great strengths was his relentlessly positive attitude, which he put to memorable effect during his bout with the demon Akaza in Demon Slayer The Movie: Mugen Train.