>>48317321That's character agency, not character in and of itself. Cynthia is a supporting character and Blue is an antagonist. Alternative formats such as the Adventures manga have muddied the waters on his role in the story but he's not a deuteragonist, he's an antagonist. Both Cynthia and Blue still have identifiable personality traits, what with Cynthia's weird way of speaking, her interest in myths, and her somewhat generic love of battle. She's not a non-character, she just doesn't play a very large role in the story.
Compare Lance who we know practically nothing about and is more or less just a generic strong guy with principles. He's way more bland than Blue and Cynthia, butt he actually fulfills a role you'd expect of the Champion and does it well, so he stands out better to you. Lance is definitely handled well, and it always bothered me that Cynthia did not help much against Team Galactic. Even when she shows up at the end, she's late.
Blue has a little bit of an arc by the way; it's not much, but I guess there is one there.