From episode 25, at the beginning of the "pirate" episodes. Kenshin is worried that he won't be able to protect Kaoru should pirates attack the ship they're on, and Kaoru is blushing at his concern for her. Of course, she assures him that she can look after herself. (If only the writers would _let_ her look after herself more often! Not that it isn't nice to let Kenshin come to her rescue from time to time....)