The Flame Hashira and him setting everyone's heart ablaze

Though the initial appearance of the Hashiras in Demon Slayer had been hazy, with very little detail on each of their personalities and life, the turn into the Mugen train arc has been deeply inscribed into the hearts of the viewers.

Just by appearing in a single arc and two episodes before that, the characterization and development of the story along with the character itself, Rengoku has created a space in everyone's hearts and arrested their adoration of him.

Viewers can never move on from the glowing, magnificent, exhilarating personality and his radiation of energy and adoration for his juniors. Rengoku Kyojiro is shown as the jovial, excited, formidable man who is the Flame Hashira, and he sets out to save the people who are stuck in the Infinity train crisis. His personality arrests the viewers.

With his positive aura, radiant smile, and vibrant flaming colors of his hair and cloak, he immediately becomes a mass favorite. His association with people, understanding and validating their feelings, always exuding a vibrant positivity is a calming and exciting aspect of his character that makes the viewers intrigued about the story that's about to unfold.

Through the development of the story, his flashbacks are shown to the viewers that throw light into his backstory. He grew up in a family where he was not validated throughout as a hashira.

Being the son of the former hashira, he would be expected to get trained by his father, but his father turned his back and wallowed into his depression when their mother passed away from an illness.

Rengoku is a self-taught, talented hashira who, nevertheless kept radiating an aura of positivity and did not let the negativity touch his little brother.

He taught himself flame breathing, and different styles to master, to become a great swordsman who can fight demons to save humanity. Despite being a jolly, outgoing, and radiant character, his backstory expresses how strong his emotional capacity is.

No matter how many lashes he received from his father, words of discouragement, invalidation, and ignorance, he never gave up on becoming who he wanted to be. Always going by the motto of, "set your heart ablaze", he kept glowing and growing a fondness among the viewers for his character.

On meeting Tanjiro, Zenitsu, and Inosuke, he immediately agrees to take them in as his Tsugukos. He called and treated them as brothers, an immediate spark of friendship and brotherhood grew among them as they set out for their expedition in the Mugen Train.

Tanjiro was flattered by his positive and exciting tone of voice as he screamed how delicious the food was, and praised them as they kept fighting with everything they had.

His appreciation and admiration root from the lack of such support he was entitled to get from his father. The positive influence of his character makes him all the more praiseworthy for the viewers.

When Rengoku was fighting Akaza, his loyalty, determination and the unshakeable character of his personality captured the hearts of the characters as well as the viewers.

Akaza with his constant manipulation to transform him into a demon proved to be in vain when Rengoku said that being human, being able to die, being able to feel everything and anything, was a privilege that he didn't want to let go of. He was honored to be a human and he wanted to die the same.

Tanjiro saw the perfection of combat and the skills that Rengoku possessed, and without interfering, let him fight head-on with Akaza because he knew his interference would only slow him down.

With unstoppable jags and blows to him, Rengoku fought without stopping, even when he lost his eye, even when Akaza induced extreme pain into his body, even when he touched the brink of death and knew he couldn't make it back, he kept fighting till his last breath.

Even though Akaza is a demon who can cure any physical hurt or impairment, Rengoku hurts him just as much, enough to make him elope the place.

However, this happens too late for Rengoku, as his pains are not curable, given that he was deeply and incurably affected and hurt, with a hole in his heart, his total concentration breathing couldn't cure it as well.

Tanjiro, Inosuke, Zenitsu, and the viewers as well break into tears, seeing him smiling even in the last moments. He promises Tanjiro that he leaves his books on flame breathing for him to find out about Hinokami Kagura.

He asks him to go to his native home and retrieve those books. Inosuke and Zenitsu join in as the help reaches them to take them back to Butterfly mansion for their treatment, Rengoku keeps smiling till the time his soul leaves his body.

He sees his mother in his vision and asks her if he was a good enough swordsman and a son. He gets his validation at last. This tear-jerker episode (Episode 7 – Set Your Heart Ablaze) gets deeply imprinted into everyone as his death puts an everlasting impact on the character and its craze among the viewers.

His death can be counted as one of the unforgettable deaths in the history of anime. Such a character and its development is a beautiful creation of the makers of Demon Slayer among the other mainstream characters.

Rengoku impacts everyone's minds and lives as the hashiras deeply mourn his loss as well. He is a ball of sunshine on an otherwise dark day, and his association with everyone without any discrimination made him an adored character among the characters and the viewers.