kaeya and diluc are siblings, and here’s why.

my first point:

the main argument that klc shippers make is that they are not blood related and that they are only ‘sworn brothers’ as said in the original chinese version of the webcomic. however, although they are sworn, they are said to be legally brothers too.

i’m not chinese, nor do i speak the language myself, but on the genshin impact fandom wiki there is a section on kaeya’s page that directly says:

‘In his Vision Character Story, Kaeya is referred as 义子 yìzǐ, "foster son", Crepus is referred as 养父 yǎngfù, "adoptive father", and Kaeya is called 养子 yǎngzǐ, "adopted son".’

so, if even in the original text’s language it says that kaeya is legally crepus’ adopted son, then kaeya is also legally diluc’s adopted brother. meaning that while they are both sworn siblings, they are also legally adopted siblings too. thus morally and lawfully, kaeya and diluc are considered siblings by the society around them.

my second point:

another argument is that kaeya and diluc live in mondstadt, a nation based heavily around germany and it’s culture. germany does not have the concept of sworn siblings, however the closest thing known is the idea of ‘blood brothers’

the ceremony, known as a ‘blood oath’, itself revolves around making a small cut on typically the finger, hand or forearm and pressing the two cuts together to symbolise the idea that each person’s blood flows in the other’s veins, effectively making them blood related.

even if kaeya and diluc did not perform this ceremony, the general consensus is that kaeya and diluc swore loyalty to each other in a means to strengthen their bond as a newly found family. diluc, who knew that kaeya was abandoned by his biological father, wanted to give him a new family who would swear to never abandon him.

my third point:

another, albeit smaller, argument is by noting kaeya’s words in venti’s story quest. while arguing with diluc, kaeya mentions off handedly that he is a little tipsy and seemingly ‘slips up’ and refers to crepus as his own father instead of only diluc’s. the idea that kaeya truly still sees crepus as his father, behind his walls of emotional defense, only adds to the argument that he is crepus’ son and still views himself as such. this implies that kaeya still sees himself as diluc’s family.