Kamiya Kaoru
Kaoru is the heroine of the story, and rightly so, for she is the strongest woman in the series. Perhaps not in terms of fighting ability (although she may very well be), but she is also the strongest in terms of her conviction, her principles, her ideals, and her heart. Kenshin's relationship with her was the most important factor that drove Kenshin to change. Though they start out as simple friends (or master and slave, maybe), their relationship slowly but surely deepened with time.
Kaoru is pretty unusual for a heroine stuck in a shounen manga. She doesn't exist for the sole purpose of being rescued. Kenshin only ever had to go and save her twice, and both times she wound up saving him. She is far from weak, not at all clumsy, is not a crybaby, and smarter and more perceptive than most of the characters around her. Her only concession to typical heroine-ness is her inability to put together a decent meal.
Kaoru's personality is immediately evident from this first encounter. She is strong-willed and short-tempered, but she is also selfless, devoted, and idealistic - all of this is then topped off by a sense of responsibility the size of the moon. Throughout the course of the manga, Kaoru is repeatedly shown as willing to risk her life to protect her students, her father's legacy, her friends, and her ideals. Obviously, she is somewhat naive and innocent, but I think she simply prefers to go through life expecting the best rather than the worst. Despite this, however, Kenshin is still able to cause her immense despair, particularly in the beginning of the Kyoto arc. Kaoru's primary concern, as she falls in love with Kenshin (so quietly and carefully, without throwing herself at him once), is keeping him sane - and this isn't anywhere near easy. She can easily recognize the change between the harmlessly polite Himura Kenshin and the perfectly lethal Hitokiri Battousai and has, at least once, forced him back from the brink of committing murder.
Kaoru's only fault besides her innocence is her very short (explosive) temper, although she makes up for this with her inhuman selflessness. She took in a Yakuza pickpocket, a street-brawler, and a full-grown yet penniless man - all with unknown pasts - without even bothering to pry into the stories of their lives. Even though she can't cook, is a bad housekeeper, and apparently keeps Kenshin as an all-purpose maid, bodyguard, and punching bag, she looks after everyone in her dojo almost as a mother would. She often stays up late to have food and baths ready for her friends when they come back after beating people up.
Although she is seldom seen fighting, Kaoru is most likely an excellent swords...er...person. After all, she is the adjutant master of her school, and also teaches outside of her own dojo, where she receives far more respect than she does from Yahiko for most of the series. Due to the very non-aggressive nature of her fighting style, Kaoru may often be viewed as weak but then, next to Sanosuke, Kenshin, and Saitou, who wouldn't be? On the occasion she does fight, against Kamatari in the Kyoto arc (vol 15), she is shown to be a calm, collected fighter who relies on wit as much as strength and skill. She possesses great knowledge about kenjutsu and can often provide insight as to why this or that happened during a fight.