She's far better as a side character because then it's easier to remain as she is. If she was the main girl, then she would need to change for the sake of story progress. Cecilia is kind of... nondescript, really. She hasn't changed at all, and honestly I don't even know how she could change. Rather than meet any particular challenges from Cecilia herself, the MC needed to conquer (convince) her surroundings. It seems like this is that kind of story, which is probably why the side characters are more peculiar and they have so much panel time.