Memory is a tricky thing: it's hard to know what really happened, what you made up, and what has been added due to other people's influence. Kathy in Never Let Me Go often uses the language of uncertainty and even admits to times when her recollections differ from Tommy's or Ruth's memories. There are times where she can't remember the reason for or location of a memory; she can't even keep them in chronological order always.
Kathy is also looking back on her life as she finishes her work as a "carer." I imagine she wants her life to look prettier in retrospect than it actually was, such as when she insists that no one cared that The Cottages were falling apart, or when she pulls out her oft-repeated phrase, "but you have to remember." She knows she's telling a life narrative, but how accurate can memory be?
Does her memory make Kathy an unreliable narrator?