One thing we come to realize after watching the Mugen Train film or the anime series is that Rengoku is the Flame Hashira because he studied Flame Breathing, but he also inherits that title from his father! In the first episode of the Mugen Train Arc, we see Rengoku on a solo mission to investigate the train that has had several passengers disappear whilst onboard. During this mission, he encounters a demon that had been killing people aboard trains and on train platforms, and subsequently deals with him. In the process, Rengoku saves an elderly bento vendor and her granddaughter, sparking within the old woman memories of having been saved in a similar manner by the Flame Hashira 20 years prior.

Rengoku’s connection to the title of Flame Hashira runs as deep as trauma and blood, as we see within his memories, snippets of verbal abuse from his seemingly downtrodden father. Rengoku’s father, Shinjuro, who is experiencing a major depression linked to his time as the Flame Hashira. Despite following in his father’s footsteps, Rengoku’s achievement is not enough to coax even the lightest compliment out of his father. Despite this disappointment, Rengoku makes the decision to dedicate himself to his work, for his younger brother’s sake as well.

The one thing anyone will first notice about the Flame Hashira is the fact that he is an incredibly energetic, enthusiastic, and excitable person whose every word is effectively an interjection. His burning soul is reflected in the flames that emanate from his sword, as well as his confidence, positivity, and almost happy-go-lucky personality.

Rengoku’s passion extends towards his work as a high-ranking agent of the Demon Slayer Corps, and also towards his title as the Flame Hashira. He puts his entire body on the line each time, confidently challenging his opponent no matter who they might be. When Akaza appears, Rengoku steps up to face him in an intense bout that could easily be considered one of the best fights of the series. Tanjiro and Inosuke notice that Rengoku is being overwhelmed by Akaza, especially considering that he’s the Upper Third-ranked demon under Kibutsuji Muzan, and attempt to jump in the fray. However, before they can even move from where they are, Rengoku orders them to stand down.

Even as Akaza tries to coax him into abandoning his humanity, Rengoku remains rooted in his duty to humanity and the young Demon Slayers behind him. Akaza’s angle was an attempt to appeal to Rengoku as a passionate martial artist, who seeks to evolve to even greater heights without limit. Akaza’s rapid regeneration only prompts Rengoku to cut harder and apply the various Flame Breathing Forms to lay waste to the demon before him, no excuses. Unfortunately, it came down to the fundamental difference between demon and human, and as most of us already know, Rengoku does not survive the bout.