Senta is a much more interesting character than she was in Brechalon. In that book she was a six year old child, and as endearing as six year olds are, they are not the stuff of an adventure book. Even at 8/9 as she is here, she is mostly interesting for the trouble she gets into. She has run-ins with wizards, a child molester, and velociraptors. But she also begins to learn magic for the first time, something that gives her more power than the average nine year old.
I think Senta gets more interesting with each book, and I suppose that's how it should be, since she's the title character. She has a destiny that she is growing into. I think she might come off as a bit of an airhead in this book, but kids are like that sometimes regardless of their potential.