Belle was a fluffy, cute kind of character, and she clicks into the story as comic relief which was a nice touch especially at the route 10 event. Unlike May in RSE, who didn't really develop much as a character, Belle really does grow up. She seems so different at the end of the game.
On the other hand, Cheren was kinda hard for me to see his character, since he doesn't talk much and doesn't let his friends in on his feelings.
Heck, around halfway into the game, I expected him to turn evil because he seemed so damn power hungry and obsessed with just winning.
Of course he meets a good mentor across the way, and shows us he has a hot heart inside his cool exterior too, but by this time Belle was the better character for me.
[edit]
The thing that most shone for me in BW is the character development/involvement in the storyline, as you can tell.
The gym leaders all have a say too. Well, except silent mask-man.
