WARNING: The following contains spoilers for Demon Slayer's "Mugen Train" arc, now streaming on Crunchyroll, Funimation and Hulu.
Demon Slayer stands as one of the most popular and best-selling manga franchises today, and could be considered one of the modern "big three" shonen titles along with Jujutsu Kaisen and My Hero Academia. From the anime's incredible visuals to the stylish, early modern setting, many factors have contributed to Demon Slayer's success.
There is another significant factor at play -- protagonist Kamado Tanjiro's kindness. Not all shonen heroes have to be abundantly generous and compassionate but Tanjiro certainly is, which helps make it clear why and how he fights. Tanjiro is battling to defend the soul of the human race, something intangible but infinitely precious to him. Several defining fight scenes in Demon Slayer make this abundantly clear.
When Tanjiro Held Off His Demon Sister Nezuko
Tanjiro's first-ever fight in Demon Slayer took place in the anime's premiere, after the archvillain Kibutsuji Muzan slaughtered the Kamado family and turned Nezuko into a demon herself. Tanjiro fought for his life to keep his sister's fangs and claws at bay, but he wouldn't ever try to kill her. Nezuko was all he had left in the world, and the strong bonds of family sustained Tanjiro and gave him purpose.
An extroverted and outgoing boy who mainly defines himself by his relationship to others and his kindness to them, naturally Tanjiro couldn't strike down his sister -- even to save his own life. Fortunately for him, the water Hashira Tomioka Giyu arrived and helped restrain her until a better solution could be found.
When Tanjiro Defeated The Hand Demon
Later in the story, Tanjiro took the demon slayer final exam on a mountain arena ringed with wisteria. In that battle zone he encountered the massive Hand Demon, a vengeful creature with the blood of dozens of humans on its hands. As revenge for its capture, this foe made a particular point of always killing Sakonji Urokodaki's masked disciples during these exams. Tanjiro slew the demon to pass the exam but also wanted to save another examinee's life, and most of all, he actually wept for his cruel opponent.
For the first time in Demon Slayer, Tanjiro truly appreciated how all demons had once been innocent humans and their humanity was not dead, but buried deep inside them. He is fighting for the intangible goodness of the humanity found in all people and demons alike, a trend that would continue later on.
When Tanjiro Threw His Sword At Akaza For Kyojuro's Sake
Demon Slayer's "Mugen Train" arc came to a thrilling conclusion when the flame Hashira Rengoku Kyojuro faced the Upper Moon Three, Akaza. Kyojuro ultimately lost his life in that fight, yet Tanjiro openly considered him the winner. Since Kyojuro died on his own terms and resisted Akaza's tempting offer to survive as a demon, Tanjiro believed true virtue won out in spite of death. He then threw his Nichirin sword at the retreating Akaza and impaled him, igniting a grudge between the two characters and proving the strength of Tanjiro's resolve.
Despite having no chance to actually kill Akaza that day, Tanjiro upheld his principles and reaffirmed to viewers that he was fighting for more than survival, revenge or even duty. For this hero, it's not enough to save lives and declare "mission accomplished." Tanjiro fights to defend the very soul of humanity -- virtues and darker aspects alike -- and he won't comprise it for any reason. That's why he defended Kyojuro so much -- the flame Hashira's flesh may have died, but his noble principles as a mortal human triumphed over Akaza's tempting offer. In that sense, the human spirit defeated the silver-tongued devil.
In Tanjiro's eyes, if he or the human race sacrifice their principles and innate goodness for the sake of survival, then their goodness really will die, regardless of what happens to the flesh. As his combat record makes clear, Tanjiro fights to rid the world of the kind of evil that will twist humanity into something it's not supposed to be. This theme may present itself once again in Demon Slayer as he, Zenitsu and Inosuke proceed into the Entertainment District with Tengen Uzui.