
I think author didn't promise anything about side couple, in s1 and s2 the gangster and mc's brother were just support characters. I'm not sure about s3 but in my opinion mc's bro didn't show any interest in gangster at all and also they don't really have interaction that much if they're alone they talked about mc and if they're not that gangster is always with mc even in their past he spent more time with mc than mc's bro it might be the mc the gangster really likes but he was ashamed to admit that he was wrong (cuz mc's bro always said gangster that he doesn't really like him and the gangster was always wanted to prove his words)
It's been so long that I dont remember, but can someone remind me if the older brother has a red string as well? Did he have a fated partner?