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I think this just breaks down to a question of definition. Nobody questions Kiara's willpower, but she has trouble containing her emotions. It's like two different aspects of what we consider to be mental fortitude. She has power, but not fine control. She's a bit like an athlete in that they obviously need a lot of willpower to get to where they are, but are not necessarily very good at maintaining their composure in public.
Meanwhile someone like Fauna has very good control of her own emotions and never lets the mask slip. This is more akin to a comedian or entertainer. They still need willpower to motivate themselves of course, but it's even more important to control the atmosphere when they're on stage.
I think it would make more sense with two different tier lists. Kiara would probably top one of them, but not the other.