I think I can reveal this without it being a spoiler, since it's not that important to the plot: Keira is a Sword Hunter, who are more focused on fighting, but she specializes in infiltration. The reason she's not as powerful as others is because of when she received the call.
The call boosts the power of the hunter when they receive it, but that boost doesn't scale. Keira was an extremely powerful child, but the boost was diminished as she grew older. As opposed to this, people like Shizune or Brandon who got the call at their human peak are considerably more powerful.