An Amazon from China, Shampoo was bound by her people's laws to kill any woman who bested her in combat. That "woman" was Ranma. Pursuing Ranma all the way to the Tendo Dojo in Japan, Shampoo was then bested by Ranma again--this time while he was a man. According to Amazon law, Shampoo was required to marry any man who bested her in combat. What she didn't realize was that the two people who had defeated her were in actuality the same person.
Shampoo has fallen in Jusenkyo's Spring of Drowned Cat, and therefore becomes a cat when in contact with cold water.