Final answer:
In the book Illuminae, Ezra calls Kady by the nickname 'Ace' as a term of endearment.
Step-by-step explanation:
In the book Illuminae, the character Ezra consistently refers to Kady with the affectionate nickname 'Ace'. This term of endearment reflects the close relationship between the two characters and illustrates Ezra's view of Kady as a skilled and competent individual, perhaps even alluding to her proficiency with computers and hacking within the narrative of the novel.
Throughout the Illuminae series, character relationships and their dynamics play a central role, and the use of nicknames is one way these relationships are depicted and explored by the authors.