Bao Si 褒姒 was a consort of King You 周幽王 (r. 781-771), the last ruler of the Western Zhou dynasty 西周 (11th cent.-770 BC). She came from the statelet of Bao 褒. Si 姒 is her family name, and her personal name is unknown. According to custom, women of the nobility were officially called with their home state and the name of their family.
Story also goes that the ruler of Bao presented her to the king when the latter attacked the state of Bao (compare the stories of Mo Xi 妹喜 and Da Ji 妲己). Bao Si, favourite of King You, gave birth to a son called Bo Fu 伯服. King You thereupon deposed Queen Shen 申后 and her son, Prince Yijiu 宜臼 (also written 宜咎), and made Bao Si his main consort.
From then on King you did not any more care for the kingdom but feasted Bao Si all day. According to legend, a smile of her would move all the world, but Bao Si refused to smile. All the king's attempts to make her smile failed. One day he made a new effort and had all fires of the signal towers lit which were installed to alarm the kingdom in case the barbarian tribes should attack. When the fires were lit, all the regional rulers (zhuhou 諸侯) made themselves ready for battle and hastened to the court to receive the king's orders - but alarm was just a play. This joke finally made Bao Si burst out laughing, to the great delight of the king.
He therefore ordered to played this game once and again. The first few times, the generals and lords hastened to the court each time alarm was announced, but then decided no to let them fool again and ignored the fake alarm. When shortly later the Western Quanrong 犬戎 barbarians did indeed attacked the border towns, and the alert torches were lit, none of the regional rulers appeared at the court.
The Quanrong overran the royal army at Mt. Lishan 驪山, conquered the capital Gao 鎬, killed Prince Bo Fu and abducted Bao Si. King You died in the battle, and his son, Prince Yijiu, was enthroned. He is known as King Ping 周平王 (r. 770-720 BCE).
A very strange story about the origin of Bao Si goes as follows: During the Xia period 夏 (21th - 17th cent. BCE) two dragons appeared in the royal palace where they left behind saliva (or, rather, sperm) that was kept in a box by the king of Xia. Centuries later, during the rule of King Li 周厲王 (r. 878-841 BC) of the Zhou dynasty, the curious king opened the box, so that the sperm flew out of it and covered the floor of the palace. The king ordered the palace girls to take off clothes and to shout at the liquid, so that it transformed into a turle that crept into the women's apartments, where it met a seven-years old girl of the court women and impregnated her.
Much later, during the reign of King Xuan 周宣王 (r. 827-782 BC), the woman gave birth to a girl. She was shocked and immediately discarded the baby. The child was adoped by a couple of traders. A prognostication warned the king of the couple, so that they had to escape to the state of Bao, where the girl was raised. The lord of Bao was once charged guilty and had to be punished by King Li. In order to escape the king's attack he offered to him the girl that had been found. She was known as Bao Si and entered the harem of King You in 779 BCE. This story, reported in the Shiji 史記 is apparently a mixture of a local tale and historical events.