It might be because comic relief. I for one remember laughing so hard at Sain I spit on my friend's GBA. That said most of them are meh to me, Sylvain is the first one I'd actually take home lol. It might be the red floof but he's freaking adorable.
Can see that it's become a bit overused though, much like what I'm gonna add:
Dead parents
Jeralt didn't have to be playable! He just needed to survive and have a good story role for longer. It's clear IS is using the dead parents as a reason to spur the main lord or avatar into action and give them a reason to hate the enemy. And there are other ways to do this, not as easy, but these are supposed to be professional writers I'm sure they can do it. I rooted for Jeralt to break this trend. I really did. Oh well he can be bros with Greil now. #DeadDadClub
Elitist players
If it counts. People say the harder modes are easy while I struggle. I made a topic about this.