One of the most defining characteristics about Kathy as a charater and how she develops from childhood to adulthood is the way in which she becomes more and more accepting of her lot in life and the kind of fate that awaits her. Whereas when she was at Hailsham, her life was filled with jealousy and envy of Ruth and her possession of Tommy, what comes to characterise her life later on is a stoical acceptance of what must be, although this is often tinged with the capacity of thinking of other possible futures. For example, examine the following quote that describes Kathy's acceptance of life and what it brings her:
We all complete. Maybe none of us really understand what we've lived through, or feel we've had enough time.
The underlying aspect of life for Kathy is the way that she knows she will complete some day, just like every other clone (and human). Other issues, such as understanding the value of our life and its experiences, and feelings that we haven't had enough time, are completely secondary to this reality.
If we examine another aspect of Kathy's character we see that when she looks back on the kind of life she and Tommy and Ruth experienced, she is very introspective and thinks about what could have been done differently:
It never occurred to me that our lives, until then so closely interwoven, could unravel and separate over a thing like that. But the fact was, I suppose, there were powerful tides tugging us apart by then, and it only needed something like that to finish the task. If we'd understood that back then--who knows?--maybe we'd have kept a tighter hold of one another.
Kathy learns through the experiences that she undergoes as part of becoming an adult, and recognises the value of clinging on more tightly to those you love in a vain attempt to try and have more time together before accepting the inevitable.