There is a legend recorded in the history compiled by the Ryukyu royal court that King Shunten was the son of the Japanese Heian period exile Minamoto Tametomo.
The story goes that Tametomo was exiled to Izu, Oshima Island after his loss in the Hogen (Imperial regency) disturbances in medieval Japan. When he tried to escape exile, he was washed ashore in the northern part of the Ryukyus by the current and by fate of heaven landed in Nakijin. For this reason the port in the area is called Untenko (fate of heaven harbor). Tametomo moved to the southern part of Okinawa and married the daughter of the Ozato Aji (chieftain) and had a son by her. He later returned home alone to the Japanese mainland, leaving his wife and son behind. His wife and son waited long and faithfully for him to return at a location near a harbor which bears the name Machinato (Makiminato), or "waiting harbor". The son went on to be King Shunten.
While it seems like a simple fable, its historical context is of note. In the 17th century the Shimazu Clan of Kyushu used this story as one of the necessary justifications for the subordination and invasion of the Ryukyus. In other words the Tokugawa Shogunate used the rationale that the Ryukyu kings, the Tokugawa family and the Shimazu Clan had a common ancestor within the Minamoto line and thus the Ryukyus could then simply be incorporated into the Shogunate system of Japan.
It was by this design that the story of Tametomo came to be written into the very first history of the Ryukyu Kingdom.