There are a number of quotes that point out Kit's impulsiveness, or tendency to act before she thinks. After she insists on making a trip in the longboat, the narrator says "she already regretted this impulsive trip ashore." A little later, when a little girl drops her toy in the sea, Kit reacts characteristically - "'Turn back, Captain,' she ordered impulsively." Kit is aware of her inclination to act impulsively, and chides herself, "I am forever doing foolish things!"
Kit is genuine and not shy or pretentious. When she meets John Holbrook, the narrator describes her direct response - "'I am Katherine Tyler,' she answered forthrightly."
Kit also has a strong, robust constitution, and is at home on the water. The author says, "she had proved to be a natural sailor...certainly she had not spent the voyage groaning and retching like some of the passengers."
(all quotes are from Chapter 1)