After Ranma and his father visited Jusenkyou, they passed through a village of Amazon women, The Joketsuzoku, high in the mountains of China. While they were watching a combat tournament, they accidentally ate the first prize. The victor of the tournament, a beautiful, violet haired Amazon girl asked Ranma and Genma violently, demanding to know why they had eaten her victory feast. Ranma, being female at the time, reasoned that if he won the tournament then the feast would rightfully belong to him anyways it would solve any difficulty. He challenged Shampoo to a duel for the first prize in the tournament.
It was tough battle, since Shampoo fought well, Ranma defeated her. Then, much to his surprise (mostly because he was female at the time), Shampoo kissed him. The Jusenkyou guide became very upset and told Ranma that Shampoo had just given him the 'kiss of death'. She would pursue him until she killed him.
Ranma thought he had eluded Shampoo by going back to Japan, but she turned up in the middle of remains of an ice cream parlor (the parlor was in one piece before Shampoo's dramatic entrance). Narrowly escaping certain death, Ranma changed to male-type, and ran back to the Tendou dojo to warn his father. He found Shampoo waiting there for him. Not unsurprisingly, she did not know who Ranma was, and was looking for a red-haired girl named 'Ranma'. Then, quite by accident, he knocked her out. When Shampoo came to be awaken, Ranma was certain he was on the kill list again. Then Shampoo grabbed him and embraced him rather fondly, much to Akane's anger.
It seems that Joketsuzoku Amazon Tribal Law requires an Amazon to kill any woman who defeats her in combat, and to marry any man who does the same. Shampoo was quite happy with Ranma, but tried to kill him again once he changed back into a girl. From the words of Soun Tendou: "When he's a man, she wants him. When he's a girl, she wants him dead."
Ranma eventually tried to stop her pursuit by claiming to be a girl in disguise the entire time she was chasing "Boy-type" Ranma. Shampoo left Japan in tears, going back to China where she stumbled upon the cursed springs of Jusenkyou herself. During training with her grandmother, she fell into the 'spring of drowned cat'. Once she realized what had happened, she mailed herself (in a small box) back to Japan, so she can try to marry Ranma. Unfortunately Ranma has a deep and abiding phobia of anything feline.
She lives and works at Nekohanten,'The Cat Cafe' with her great grandmother, Cologne. Cologne is the leader of Shampoo's tribe, and she is a fearsome, though incredibly short, martial artist.
Remember, Shampoo always speaks in third person.