One thing I’ve noticed about Bianca and Cheren: Bianca always ends up being the more useful of the pair. In the Relic Castle sequence, Cheren just tags along behind you, ultimately adding nothing to the situation. Bianca, meanwhile, gets ahold of Juniper–which turns out to be really important since they find the dark/light stone. In the Elite Four sequence, the same thing happens. Cheren tags along and beats the Elite Four as well (not contributing much of anything to your predicament) while Bianca rounds up all the Gym Leaders (who save your ass). I think this was probably intentional, and it sheds light on how the writers wanted us to view Bianca and Cheren.
Hmm. I think that’s a little unfair to Cheren; he does fight alongside you against Team Plasma on multiple occasions, and fighting usually makes up most of the player’s contribution to advancing the plot. And I don’t… think Bianca is responsible for getting Professor Juniper involved in looking for the Dark/Light Stone, or at least I don’t believe anyone ever says that’s what she’s doing. I’d be more inclined to assume that that was the elder Professor Juniper, who is present at the Dragonspiral Tower when the player confronts N, and works together with his daughter to identify the stone. There is a general point to be made about Bianca and Cheren as foils to each other, though. The early part of the game kind of sets up Cheren as more organised, more ambitious, a better trainer, more… well, frankly, more competent, whereas Bianca doesn’t really know what she’s doing or what she wants. Over the course of the game, though, Cheren comes to realise (through Alder’s example) that his ambitions are basically hollow, leaving him somewhat listless at the end of the story; Bianca, on the other hand, grows into herself, figures out what she wants to do with her life, and becomes a researcher. She’s ultimately the one who comes out of it with a stronger conception of her own goals and identity. I think the message is supposed to be about taking time to explore life, and figure out what your goals are gradually and organically, rather than focusing on the single-minded pursuit of just one aim in the belief that it will complete you as a person (Cheren actually credits Bianca, as well as the player, Alder and N, with helping him realise this).