Himura Kenshin's darker personality, the Hitokiri Battousai breaks free whenever the rurouni is under a great deal of stress -- kind of like a Meiji Era Incredible Hulk (he even does the yellow eyes, but draws the line at the icky green skin). He is stronger, meaner, sexier, and far less polite than Himura's normal self: quite definitely not a force to be dealt with lightly.
In most circumstances, the only way to awaken the Battousai is to threaten the life of someone Himura has decided to protect; however, when it comes to his landlady, even looking at her wrong is grounds for beheading. This process is referred to as "going Battousai", and while Kamiya Kaoru is firmly against killing, protective anger suits her just fine. In fact, it seems likely that the only way she'll ever get some action is to spend quality time with the hitokiri.
(note: Definition shamelessly stolen from h2g2.)
Kenshin: "No. I believe that a sword is a weapon, and Kendo is a technique for killing. Miss Kaoru believes that I cannot kill a man... (smiles) I just love her naive impression of me! I wish that it's the truth, but I'm afraid it isn't."