Why do you think staying friends with Ruth is so important to Kathy?
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Ruth is important to Kathy because she's one of her only friends from Hailsham which means Ruth is also one of her links to remind her of Hailsham once they move to the cottages. This also answers your question of why Kathy makes excuses for Ruth. Ruth is her only friend, her link back to Hailsham and also, Ruth can be quite manipulative so Kathy may have felt she had to make excuses for her.
In the cottages we can see that Kathy is feeling pretty sad about Ruth finding new companions and she is left alone. Kathy can't let go of what she had at Hailsham and clings to the memory and emotions of their past friendship, and uses this to encourage her to maintain that friendship in thought and in reality. Even if she knows she will come back to her in tough times