Kestrel was one of the main characters in The Wind Singer by
William Nicholson, so yes
Kestrel sounds good as a fantasy character. I think
Kerensa also works as it is another less usual name.
I think using them as twins would be better than friends because it is probably more likely that twin names would begin with the same letter rather than friends.
As for the kind of person that comes to mind I obviously think of the
William Nicholson character for
Kestrel however, for
Kerensa, I seem to picture I meaner character.