Toswiah/Evie guides us through her story, telling us what happens to her and around her, which is how we know she's a first person narrator. Because she's also the protagonist, she's a central narrator.
However, Toswiah/Evie isn't your standard, "A happened, I did this, B happened, I felt this way" narrator. Nope—she's a person in the middle of an identity crisis, which in her case is a literal identity switch. She's trying to hold on to her old self while building a new one, which can create some serious internal tension, as she describes in the first chapter:
I can never tell anybody the real truth. But I can write it and say this story you're about to read is fiction. I can give it a beginning, middle and end. A plot. A character named Evie. A sister named Anna.
Call it fiction because fiction's what it is. Evie and Anna aren't real people. So you can't go somewhere and look this up and say Now I know what this story's about. (1.26-27)
What Toswiah/Evie tells us here is that she's constructing a story, making sense of what happened to her through writing. Lucky us, we get to read her words.