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This lovely young lady is Ukyo Kuonji, okonomiyaki chef extraordinarie and formerly Ranma's "cute" fiancee. Wanna find more out about this girl? Well, I'm only too happy to oblige you.
Ukyo's story begins when Genma (Ranma's dad) and a buddy of his made a deal. Genma would promise that Ranma (then a little kid) would marry Ukyo (who Ranma thought was a boy), and Ukyo's dad would put up his okonomiyaki cart as a dowry (by the by, okonomiyaki is a japanese pizza-like dish containing a mix of various meats and vegetables). But, Genma, being the evil, evil man that he is, asked young Ranma which he liked better, Uyko or Okonomiyaki. To this five-year old Ranma replied "Okonomiyaki!", and thus fate intervened. Genma grabbed the cart and took off. Ukyo became the laughing stock of her villiage and went to train by the ocean, swearing vengeance.
So, way later on, Ukyo shows up (dressed as a guy, because, of course, Ranma still thinks she is one. Silly Ranma.) and she's got it in for Genma and Ranma. Ranma discovers Ukyo's a girl and that she's technically his fiancee, and calls her "cute". Well, that pretty much does it. Ukyo falls for Ranma in that moment and spend the rest of her time from then on trying to get Ranma to fall in love with her.
When Ranma Saotome finally declared his intent to wed Akane Tendo, the fragile pseudo-world that Ukyou Kounji had built for herself in Nerima came to a crashing halt. Desperate to forget the painful memories of a year's life basically wasted, she goes for broke, and decides to accompany the Lost Boy Ryouga, with whom she had schemed to seperate Ranma and Akane, several times before. She doesn't know where the road will lead her, but anything is better than the memories.
Uykou's slowly kindling romance with Ryouga Hibiki has blossomed into love, and she's now his informal fiancee. All she needs now his her father's approval to make the wedding concrete.

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