charbios: Motoko Aoyama
She has difficulty dealing with other people, especially males; she finds men to be a distraction that keep her from being able to concentrate fully on her studies and kendo practice. Her relationship with Keitaro initially resembles Naru's relationship with him; he, all too often, finds himself on the receiving end of various attacks as a result of his frequent mishaps.
She eventually mellows out as she becomes used to him and eventually finds herself developing feelings for him, along with a habit of writing trashy romance tales involving the two of them. In fact, she is the first of the girls (aside from Naru and Mutsumi) to confess her feelings to Keitaro.
As time passes, she fails the Tokyo University entrance exam herself and becomes a ronin as Keitaro had been. The irony of her situation and its similarities to his was not lost on her. She eventually passes the test, and becomes a law student at Todai.
Motoko has an older sister, Tsuruko, who gave up the life of the sword to marry some guy. Motoko lives in fear of her sister, though the problem is primarily in Motoko's head; Tsuruko is not nearly as severe and frightening in reality as Motoko remembers. It's interesting to note that Tsuruko is actually Motoko's biological sister. Motoko and Su are the only characters in the series who are actually blood-related to their siblings.
Motoko is probably one of the most fully developed characters in either the anime or manga; the calm, almost serene Motoko who appears at the end of volume 14 of the manga release is quite a different person than the hostile, repressed individual Keitaro encountered at the start of the series.
Very stoic and serene, Motoko is wholly dedicated to carrying on the family tradition of being top-class swordwielders. Even at her young age, she's considered one of the best, and she moved to Hinata from Kyoto to attend a school renowned for its kendo club, just to make sure she stays in top form. She practices the art of Shinmei Ryuu and can channel her soul energy into her swings.
Part of her drive is the fact that she's not too fond of her older sister. Motoko feels that she had betrayed the family by not carrying on the tradition herself: instead, she fell in love and got married. As a result, she has forsworn men. This makes Keitaro's presence at the inn a threat to her calm and concentration, so she's frequently taking swings at him. She also can't stand Onsen Tamago (she's terrified of turtles).
Naturally enough, romance is her big failing. Not only can't she stand men and spends all her time in stoic concentration, but she's tall for her age (in fact, she's the tallest person in the entire inn, Keitaro included). Her manners are also a bit archaic: she's all prim and proper, just about 140 years out of tune.