I'm sure the other different pairings like Seri and her roommate, or Sakurako's sister and her roommate, all have their own flavors and nuances. I just can't remember them after following this series for so many years lol.
Shouko and Seri have what is probably the most "basic" I can't think of a better word romance relationship in the series. Two opposites who just happened to be roomed together and after a while found themselves attracted to one another and ultimately acted on those feelings. Not gonna lie, the chapter where they reveal those feelings to each other is probably one of my favorites in the series.
Far as Sakurako's sister and her friend, I don't think we've actually seen enough of them to really call their relationship anything other than a friendship. I could be wrong there, though.