The tone in Never Let Me Go is reflective and introspective, often laced with self-doubt and uncertainty. Kathy H. serves as the narrator of the novel and, as such, her emotions and fears largely influence the shifts in tone within the narrative.
Kathy spends much of the novel looking back on the past, reminiscing about her days at Hailsham or the Cottages. Many of these remembrances are voiced in an uncertain tone, Kathy herself failing to recall the details of one circumstance or another. Note this uncertainty in the following passage from Chapter 16:
I went through a spell of suspecting that [Ruth discovering the cassette] hadn't been by chance at all; that she'd noticed it there maybe days before, perhaps even examined it to make sure, then had waited for the best time to "find" it. Years later, I gently hinted this to Ruth, and she didn't seem to know what I was talking about, so maybe I was wrong.
Kathy doubts her own perspective, narrating her memories in a tone laced with uncertainty. She cannot trust herself, given that past and present trauma cloud her mind, stifling remembrance. Kathy's identity has been reformed and reshaped, stripped from her, and centered on her social "role" as a human clone and organ donor. An uncertain tone is appropriate for her character, taking into account the extent to which her humanity and perspective have been undermined by society.