Which pairs changed the way fights are designed?
One thing that comes to mind is the enemies gaining status shield upon recovering from sleep and other statuses that have a time limit.
To this day I think it was added as a way to counter Serena. For those who didn't play back then or don't remember, Serena's sleep + Go Viral 9 trivialized almost every fight. You'd use her and whatever strong unit you had (SS Red was a common unit used) and just sleep lock the enemies. With this you could complete Vila with just the three pairs.
Sure, Agatha could also be used for the same thing, but with Serena, you didn't have to change targets and time 3 enemies at once.
This changed with Legendary Arena, where enemies would start to resist status condition when they are applied multiple times. And was ultimately crippled when status shields were added.
I was wondering if there are other cases where a strategy is so strong, almost every fight needs to have a passive or gimmick that would counter it.