Karen and Claire. Should I leave them or should I change one because they have too much of the "air" sound to them?
I would say that yes, they do sound too similar. How much of an issue this poses really depends on the importance of each character in the story--would use of both names allow for too much confusion to the reader? I think you should test it to be sure.
I disagree. They aren't too similar - different number of syllables, different starting letter. Readers won't be confused.
It all depends on the relative placements of the characters in the novel, and how distinctive each character is apart from the name.
I'd say no - but I don't hear this "air" sound in "Karen" that you're referring to. It's pronounced like "Kah-ren" where I'm from, so I'm assuming it is pronounced differently where you are?
I don't think it'd be an issue. In "One Hundred Years of Solitude", the author named a million people the same name, which, admittedly, got extremely annoying. I had to go back and forth one hundred times to keep everything organized...but I still considered it a great story. You should change them just to be safe though, since the names don't seem to be all that important to you or the story.
Great if they sisters not so great if they are love rivals. As long as they are easy to differentiate via character I don't see a problem. I say love rivals as there would be a lot of bouncing around between these two names I then i can get confusing.
