You’re the loneliest person I know.
-- Hernandez (chapter 1)

Importance: Hernandez says this while speaking to Cassie in the first scene. This interaction specifically highlights the fact that Cassie leads a generally isolated existence. Due to the trauma she suffered as a teenager, Cassie is generally averse to romantic and sexual intimacy. Thus, she shields herself from relationships that could potentially become intimate, and she leads an often unfulfilled life.

Am I guessing right that [Thompson] has something to do with why you never date anybody?
-- Hernandez (chapter 2)

Importance: Hernandez asks this question following the incident in which Cassie hits Thompson. Cassie nods in response to Hernandez's question. This interaction is one of the novel's firs heavy implications about Cassie's past. When Cassie was 16 years old, Thompson raped her. The traumatic experienced emotionally scarred Cassie and caused her to be afraid of romantic/sexual intimacy.

...women in the fire service will be the downfall of civilization...
-- Captain Murphy (chapter 5)