Demon Slayer Fans Can't Get Enough Of This Rengoku And Tanjiro Detail
Anime isn't necessarily a medium known for subtlety. Often times, the story told therein are intentionally direct about their morality or themes. Consider the oft-forgotten sci-fi comedy "Assassination Classroom," which is literally about an octopus monster teaching valuable life lessons to a group of normal middle school students who are tasked with assassinating him before he blows up the earth. Aside from the blood and stabbing, each episode is wrapped in with a summary that wouldn't be altogether out of place in "VeggieTales."
So it wouldn't be surprising, then, to expect a similar level of subtlety from an anime called "Demon Slayer" which is about, shockingly, demon slayers. In brief, Tanjiro is a young boy who lost everything to demons, and the one family member who survived the horrendous event was turned, vampire-like, into a demon themself. With what's left of his life, and his demon sister in tow, Tanjiro becomes a demon slayer to make sure no one has to suffer like he did. Also, he's got cool magical powers because that's just how anime works.
To assume, though, that there's no layered storytelling is going on, would be a mistake. The devil (heh) is in the details, and fans are still relishing the attention that went into crafting "Demon Slayer." With that in mind, let's take a closer look at two resident pyromaniacs, Tanjiro and Rengoku.
Rengoku and Tanjiro use sun breathing differently
In a subreddit dedicated to the series, u/TheOtherSide210 posted a photoset that depicts Rengoku and Tanjiro using their fiery powers, along with a caption, which read, "now I can see a difference in their flames." The images in the photoset show that Rengoku's flames are bulkier, with sharper lines and stark color variants, while Tanjiro's flames are more slim, with flowing curves and blended color gradients. While not explicitly stated, it's likely that the distinction is caused by their preferred breathing styles. For clarity, in the lore of "Demon Slayer," a breathing style is the magical base a Demon Slayer builds their weapon skills upon. Rengoku naturally uses Sun Breathing, which is inherently flame based, while Tanjiro naturally uses Water Breathing, which is inherently aquatic.
In media, water thematically associated with softness and curves while fire is more often depicted as overwhelming, unfeeling power. Consider "Harry Potter and the Goblet of Fire," which features a sequence where a group of strong, rugged wizard men showing off fire magic. The magic isn't even remotely relevant to the movie, it's simply used to give the audience a visual understanding of the characters involved. In addition, consider "Avatar: The Last Airbender," which not only features character who equivocate fire with rage but goes even further to show off a Firebender (read: magic fire user) using moves inspired by Waterbending (use context clues, there are various bending styles). The skill is seen as a startling deviation from normal Firebending, and the characters involved specifically mention this.
All this to say that there's solid evidence for "Demon Slayer" having done this with explicit intent in mind. Rengoku is a "pure" fire, while Tanjiro is a well-rounded one.