You killed yet?...No. I knowed you was a virgin the day we took you on.
-- Oldham (chapter 1)

Importance: Oldham says this to Callum as Callum attempts to defend Ava from Oldham. Oldham's comment establishes the idea that Callum is generally innocent compared to the Colonel's men. Callum's character arc is therefore defined in part by a partial loss of innocence over the course of the novel.

Her name is Ava. I saved her.
-- Callum (chapter 1)

Importance: This statement establishes Callum's original view of Ava and his sense of loyalty towards her. Ever since Callum's first interaction with Ava, Callum feels compelled to act as Ava's protector. This sense of loyalty implicitly conveys Callum's sense of righteousness and perhaps a nascent sense of love towards Ava.

I did what I had to.
-- Callum (chapter 6)

Importance: Callum says this to Ava after he retrieves their stolen horse and belongings. Callum did not want to kill the thieves, but when they...