Just because he's getting called "nii-chan" doesn't mean necessarily mean they're blood related as the manga already stated, nor is it inherently bad to use/do so, asians often associate extremely close friends with familiality, (e.g. childhood friends.) Literally words like hyung, gege, nii-chan/san, kuya, etc... depending on your country/nationality that pertains to an older brother can be used on older close male friends. It really doesn't take THAT much to understand that, especially if you've been exposed to asian media for a long time, considering that you're here, in this website, reading asian media.
oh God, I love u already! You're so nice for explaining the whole thing(⌒▽⌒)
Ig its because no one ever see the use of nii-chan in this context perhaps? Opposite of "aniki" which is like the same thing but perhaps "nii-chan" is more commonly use for siblings context, its understandable for some to be uncomfortable with it (●'◡'●)ﾉ
maybe its because the mc says that he thinks of him as a brother
Why is this so much better than the anime/movie. Tsuzuki looks so much better and feels fleshed out here i'm sorry