This is my best chance of explaining why I like Kaoru so much. I'll work primarily from the anime, as I prefer Kaoru's portrayal as a character in it and I know it the best.

She's got a quick and violent temper, of course, but she tends to forgive rather quickly as well. She's very trusting, too much so on occasion, and generous; she may fume about Sano's freeloading but she doesn't kick him out, either. Her emotional strength is never undermined, even when her physical strength is. She's intelligent, although that's sometimes forgotten for reasons of comedy. In one filler-ep scene from the anime that I really disliked, she bandaged someone's entire head, and when he struggled, she only pulled the bandage tighter, telling him to stop making such a fuss. She's not like that dammit! On the other hand, she is able to read. I'm not entirely sure how significant this is. At the time, this would probably have been rather unusual; universal education was not the norm prior to the Meiji era, and schools were set up hastily and with little attention to qualifications in the years that Kaoru would have been learning to read. However, she was a member of the samurai class, and therefore more likely to be literate anyway. Back to the first hand, education for women was different up until World War II. Would it even have existed at a time when even most men didn't learn to read? And how much is Watsuki-sensei even abiding by all of this anyway? He stated (in a freetalk somewhere) that he didn't want to create strict historical fiction, just an entertaining story with something of the feel of Meiji Japan. Soooo I'm probably obsessing over nothing.

Another note: I don't think Kaoru ever uses an honorific with any of the dojo's guys. She does use the standard -san (for Tae and Megumi [who doesn't deserve it]) and -chan (for Misao and Tsubame... and Ayame and Suzumi, of course) but with her male friends and her rival, she doesn't. The lack of an honorific can mean brusqueness, condescension (Umi, the snooty Magic Knight, doesn't use an honorific either), closeness, or a desire to keep your relation to that person vague by not overtly stating how you view them. A wish not to reveal how you stand with regard to a person is a very real possibility where Kaoru's concerned. She's fairly secure, but I suspect she's also rather easily hurt, from the way she hits people for teasing or causing her embarrassment. Her attachment to Kenshin causes her problems; she's not really used to relying on someone who might just leave, and she's not accustomed to her cooking mattering, or to any of the other issues of femininity mattering, either. But then later on, she begins to realize that they don't.

Watsuki-sensei commented, at one point, that he was trying to draw Kaoru cuter, so he kept changing the patterns of her kimonos. He said it looked like she was trying to dress herself up, even though she's "poor and unrefined." The words sound slightly negative, though he seems rather fond of Kaoru herself. But they're also fairly accurate. She is poor, running a dojo with only one, non-paying student. She teaches at other dojos and such, but she's still got to watch her money. And as for unrefined? Well, no, she's not some cultured, graceful ice maiden (I don't think it's a spoiler to say I *don't* like Tomoe.) She's not refined. She's spontaneous, friendly, tomboyish, and not overly polite. I suppose you could say she is dressing up within the manga, and if so, it's probably for Kenshin, ne? I don't think it's a stretch to say she worries about catching his attention. Yeah, the idea of him reverting to Battousai or getting killed is a heavier worry, but she's not above the more mundane stuff. She's also able to put aside mundane things. At one point she tells Kamatari that when she's fighting, she doesn't think of herself as a man or woman, just as a fighter. Ain't she cool?