If you go with the suggestion of
Katherine, you'll have to check up on nickname usage in the period in which your story will be set. (You shuld probably also use either
Catherine or
Katharine, but not a hybrid - too modern.) The 1800s took a long time! At the beginning she'd quite likely have been
Kitty, then
Cathy later and
Katie later still. Depends on how scholarly you intend to be, too - if it's just a costume drama, then go with whatever pleases you.