This is a big question, one that drives a lot of the novel's tension. First of all, if you haven't already done so, you should definitely read Ishiguro's Never Let Me Go because it is a beautiful, powerful, and strange story.
Kathy is the first-person narrator of Never Let Me Go, so right away Ishiguro gives us an intimate connection to Kathy, as well as seemingly complete access to her thoughts. Kathy controls how we view the entire story and its characters, including her own character.
Kathy's POV, however, can be frustrating at times. Ishiguro, through Kathy's narration, depicts Kathy as rather hesitant and unsure. Many times throughout the story, Kathy doubts the quality of her own memory, telling us that she can't be sure the way she recalls something is how it actually happened. Kathy's actions and life events, however, starkly contrast with her hesitant storytelling—Kathy is one of the oldest carers, and therefore one of the story's survivors (again, if you haven't read the story, I really don't want to spoil it for you by revealing more!). Kathy also describes to us the decisive way she moves forward in her life, telling us,"...I made my decision, and once I'd made it, I never wavered. I just got up one morning and told Keffers I wanted to start my training to become a carer." Thus, Ishiguro brilliantly places Kathy's narration at odds with her actual actions, disorienting the reader and revealing Kathy's unassuming and self-effacing nature.
Though Kathy seems to lack confidence in her storytelling at times, the events of the story and the ways in which Ishiguro contrasts her with the other characters, also reveal how stoic and strong she is. Because of her stoicism and strength (and because of the nature of the world in which the book takes place) Kathy tends to downplay the horrific details of some of the story's events. This tends to throw us readers off, and as a result, it takes us a bit longer to realize how terrible the story's situation really is.