It's A Metaphor

Ryder Banks is a sociopath. Yet, she finds to call herself a creature. She still can do normal teenage things, but to be in a mental facility since you were five has a strong effect of you. She has only one friend; Levi Riverside.

Levi Riverside has anxiety and strong depression, but to Ryder, he's a beauty and she's the beast. He rarely talks yo anyone except Ryder. She's like his translator, yet they completely understand each other.

Metaphors are the way she describes herself and others around her.

Head movements and sighs is how Levi describes others and talks to people around him.