Final answer:
In the novel 'Illuminae,' Kady Grant is the character who has a personal journal, which serves to deepen the reader's understanding of her character and offers a unique perspective on the events unfolding within the story.
Step-by-step explanation:
In the book Illuminae by Amie Kaufman and Jay Kristoff, the character who keeps a personal journal is Kady Grant. This journal is significant to the narrative as it provides insight into her thoughts and feelings, and helps develop her character throughout the story.
It is through Kady's journal entries and other multimedia forms of storytelling, such as emails, interviews, and reports, that the reader experiences the complexity of the plot and the development of the characters in this science fiction novel.