The author reveals: What Karla says, does, and believes.
Karla's individuality is introduced to the reader by the author by illuminating her thoughts, words, and deeds.
For instance, there is a scene in the novel where she considers her cousin's nonverbal cues. Karla imagined her cousin shrugging as she turned away from Jade and proceeded down the hall to math class.
In addition, we can see how she inhibits her own learning. "It's somewhat like arithmetic, but not quite," she mused in silence. My body must be connected to my brain so that I can control when to move or halt it. Here, it's important to carry out each action deftly."