@tanjiros-haori exactly exactly! a lot of the found-family headcanons in fandoms is to put the characters into neat nuclear family boxes—and the complexity doesn't get acknowledged beyond that. i've seen 500x more "they're so brothers!" than any word about the effort that existed from both sides into creating the brotherhood in the first place (in the context of giyuu and tanjirou). of course it's not inherently inferior to believe they're brothers and that's why their relationship works if you aren't specifically a fan of them. i know i sure don't go into much depth on the internet about characters that i care little about. but that as a headcanon is so widespread its basically the default, which is wild to me because i've never seen them as brothers. fandoms do alter your perceptions, sure, but it's unbelievably ubiquitous, which is why i made the original post.
to tanjirou, giyuu is his benefactor first and foremost. he was the one that not just believed him and believed in him since day 1, but also had expectations. in demon slayer at least this kind of expectation-responsibility imposed on someone is consistently portrayed as necessary. urokodaki with tanjirou, later rengoku with tanjirou, ubuyashiki with gyomei & the corps, shinobu with herself and kanao, etc.
and beyond that, to tanjirou, giyuu is his savior, his hero, his senior, and the hashira/the most skilled user of his own breathing style. after the four days in HTA, giyuu is also someone that's not just important to him due to the demon slayer duties, but also someone he relates to on a personal level, with a tie to the past connecting them through sabito and experience.
to giyuu, tanjirou is someone who he placed all his expectations on. the one he expected would be better than himself to take over his duties. simultaneously, tanjirou and nezuko were special in a way there was no record of, or at least none that he knew about first hand. it's no wonder giyuu immediately hailed tanjirou & nezuko as the chosen ones.
and that aside, tanjirou too is someone giyuu feels for. hell, tanjirou's proposal for a soba contest, that involved two things he loved: food and competing, it was so perfect it got giyuu to want to spend more time with him, just as friends. it was the kind of friendship he needed the most. i won't get into this much but this is where giyuu's themes of protection of tanjirou shines the brightest mirroring akaza's, obviously that makes tanjirou important to giyuu.
these are things you just cannot box into a label, at least to me. there's so much more going on than just them liking each other and declaring the other as a brother. hell, i don't even think they're aware of the depths of the other's feelings for them like us as viewers are. they can infer the other likes them, but there's no explicit expression of that. i don't think that's brotherly.
the hinokami chronicles game touched on this a bit with tanjirou crying over words as simple as "you've gotten stronger" from giyuu. he craves that kind of acknowledgement from someone who he pretty much idolizes in strength. repeatedly we're told that tanjirou thinks highly of giyuu and wants to use that belief to not let him or anyone down (entertainment district arc and swordsmith village arc). but hearing it directly from giyuu is different. this, to me, is pretty much explicitly a senior-junior relationship. i mean yeah of course younger siblings would too crave acknowledge from elder siblings, but the way giyuu and tanjirou see each other as not familial but rather a right-place-right-time give-and-take relationship i interpret them as seeing the other as friends more than as family. i don't think one has to be superior to the other, especially with the sheer level of love and respect and belief they have in eachother.
tanjirou and giyuu did change eachother fundamentally. hell i go as far as to say giyuu's entire character revolves on tanjirou's like a planet to the sun. or rather if i HAVE to use an analogy it'd be like one of those smaller planets with a big ass moon that also influences the gravity of the said planet like pluto with its moon or something. i don't know what im saying but i hope you get me lol. demon slayer from start to the end is giyuu and tanjirou influencing each other constantly, sometimes the reason the plot exists (chapter 1, 1st hashira meeting, akaza vs giyuu/tanjirou, chapter 201 with DKT) and other times nurturing eachother in a substantial way (giyuu's backstory, the sanemi ohagi thing, chapter 199 red blade).
like you said, "cause it's "deeper" but isn't always", i don't think labelling them strictly as brothers makes their relationship deeper.
something regarding giyuu & tanjirou in my view is i just cannot compare them to most siblings in demon slayer even if i make myself view them as brothers. the closest is masachika and sanemi & shinobu and kanao. more than that i compare them to relationships like obamitsu (for unpleasant reasons, i'll get to that) and hakukoyu (for much more pleasant reasons). i think the main reason i can compare and contrast giyuu and tanjirou's relationships to relationships so dissimilar to their own (i.e. even romantic relationships) is because they fundamentally represent love and humanity in line with the wider themes of demon slayer (which could be a whole post in itself really), and more specifically because they aren't boxed into a "type" of relationship.
regarding obamitsu, i'm sorry, i don't really like that ship in canon, and giyuu and tanjirou's relationship serves as my standard for why i don't. i'll explain. obanai and mitsuri are both hashiras, and more than that, they're meant to be romantic. but while giyuu and tanjirou, 2 different positions in the corps with giyuu being the senior, are treated as equals and respected equally by eachother, obamitsu's major scenes have mitsuri seeing obanai as a strong guy, which i really really don't prefer. i wish they were treated with the same respect giyuu & tanjirou have. despite the difference in positions, both of them in the infinity castle took a moment to fawn over eachothers' strengths, something we definitely don't see from obanai towards mitsuri. their scenes at the start of infinity castle arc are RIGHT next to eachothers which makes this difference so very obvious.
in contrast, hakukoyu is perfectly done, like giyuu and tanjirou. both the duos are portrayed as mutual salvation. there's themes of equal nurture. for giyuu and tanjirou that's mostly DS corps related, for hakukoyu that's related to life itself, while the same can definitely be said for giyuu and tanjirou. in fact giyuu's "you have to get through me first" speech is what unlocked koyuki's memories in akaza. to akaza, in that moment, giyuu was to tanjirou what he himself was (supposed to be) for koyuki. the want to protect someone not just out of morals but pure devotion towards them, which for akaza it's romantic devotion towards koyuki and for giyuu it's "friends and family" kind of devotion (his words saying "i wont let friends and family die before my eyes again" referring to tanjirou), which, at the same time, we are shown tanjirou and koyuki's respective devotions to giyuu (protecting him again & again in the same battle) and akaza (waiting all this time for him to stop and remember) too. it's all mutual. this whole fight is so good and its because all sets of relationships (giyuu - tsutako, sabito, tanjirou // akaza - dad, keizo, koyuki) are done so well.
you can infer generally that they're senior and junior, giyuu as tanjirou's benefactor and tanjirou in-turn reminding giyuu the value of his own life, things like that, but there has never been a time any source from gotouge or ufotable has boxed them as anything particular.
of course its fair for the fandom to see them as brothers, that's not the issue. it's rather that all of what they *are* is packed densely, labelled as "brothers", and that word itself—rather than anything that the label encapsulates—is used as the emotion-driver or "angst" for the two, and i think that's horribly unfair to the depth of writing they have.
i'm sorry i know i talk too much when it comes to them lol
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