It' s not that way. I think it comes from korean "hyung" witch is a respectful way to call a older man who is your close friend.
It's because there are no such honorifics in the English language. I'm chinese and we call older boys ''gege'' for example (they have to be ''close'' though). It's part of east-asian culture! Since this manhwa is a historical one, Rishar can't call Etienne ''Hyung'' . Instead they use ''brother''. I get you for being weirded out though haha
Not just East Asian, all of Asia actually. Southeast and south Asians have honorifics for older and younger ppl
I know they arent like blood related but when rishar called him brother it icked me out does he continue to call him.brother n like familial related titles? cuz I really wanna read this but I don't like how it give them a familial connection even if not blood related ಥ_ಥ