Nate and Chuck are best friends.
Serena and Blair are best friends.
They are all best friends because Blair gets what she wants and she wanted Nate Archibald. So naturally the other two followed suit and everyone became friends.
Chuck and Blair are the devils of the group.
Serena and Nate are the angels of the group.
Chuck and Blair are smarter and more cunning that Nate and Serena.
Nate and Serena are nicer and more well rounded than Chuck and Blair.
Chuck and Blair wear more put together outfits.
Nate and Serena look put together when they just throw clothes on.
Serena and Chuck are more cut loose.
Nate and Blair care about how they are socially seen.
Nate and Serena can be best friends and exes.
Chuck and Blair can only be completely in love with each other and nothing less.
The sex between Blair and Nate is mediocre and if I had to guess the same is for Chuck and Serena.
The sex between Chuck and Blair is explosive and the sex between Serena and Nate is just hot.
But all together they work like clockwork. They can take down anyone just as easily as having a fun conversation over brunch. They know the roles they play and that no one could replace them in the group. They might get in fights but they never last forever and always are friends through thick and thin. They are each other’s exes, spouses, gfs/bfs and even siblings but they are always friends through and through.