Hi was one of the empress's title in Kokyu palace (which is an imperial harem including the empress's residence). Under the ritsuryo system (a system of centralized government based on the ritsuryo code), Hi was the second highest title after Kogo. In modern age, Hi is used for all spouses for male imperial family except Emperor (like Kotaishi hi [crown princess] and shinno hi [imperial princess]), but this section explains an original usage of empress's title.
Hi' originally had same meaning as 'Tsuma' (wife), but later Hi was used as a title of emperor's spouse in China and this was adopted in Japan since the ritsuryo system was introduced from Tang. Unlike Okisaki (an Empress) who is a lawful wife of Emperor and has long history before the ritsuryo system introduced, Hi was considered as 'a employee of Kokyu palace' who fulfills a role as Emperor's spouse provided in Gokushikiinryo (後宮職員令) (The Administrative Code of Kokyu palace). Therefore, Gokushikiinryo and "the Engishiki" (an ancient book for codes and procedures on national rites and prayers) provide working conditions as the employee of Kokyu palace. In accordance with "Nihonkoki" (Later Chronicle of Japan), Imperial Princess Asahara and Imperial Princess Oyake who didn't follow the retired Emperor Heizei at the Kusuko Incident, resigned Hi and left the retired emperor.
Gokushikiinryo provided that the number of Hi was two at once and they must be higher than Shihon (the fourth-ranked Imperial Prince), it means Hi was only granted to Imperial princess. Gokushikiinryo also provided compensation based on Hi's honi (court rank) and bonus in spring and fall. In addition, "the Engishiki" provided granting seasonal clothes in spring and fall to Hi.
Officai history confirmed that there are many Hi in ancient times, certain ones were Imperial Princess Sakahito (Imperial Princess of Emperor Konin, younger maternal half-sister of Emperor Kanmu), Emperor Kanmu's empress, Princess Asahara, Emperor Heizei's empress and Imperial Princess Oyake (both Imperial Princesses of Emperor Kanmu, younger maternal half-sister of Emperor Heizei), Princess Takatsu (Imperial Princess of Emperor Kanmu, younger maternal half-sister of Emperor Heizei) and TAJIHI no Takako (daughter of TAJIHI no ujinokami, upgraded from Fujin [consort of emperor]), both of them were Emperor Saga's empress, Imperial Princess Ishi (Imperial Princess of Emperor Koko, aunt of Emperor Daigo), Emperor Daigo's Empress, and as wife of prince (crown princess in modern time), Princess Ahe (Imperial Princess of Emperor Tenchi, aunto of Prince Kusakabe, but Prince didn't ascended the throne because he died), Prince Kusakabe's wife, and Imperial Princess Koshi (Imperial Princess of Emperor Kanmu, younger maternal half-sister of Imperial Prince Otomo, but the princess died before the prince ascended the crown prince and the throne), Imperial Prince Otomo's wife. In addition, FUJIWARA no Tabiko (consort of Emperor Kanmu, conferred when she died), was granted the rank of Hi.
There were two Hi who were from the subjects, but both of them were promoted from Fujin. One was FUJIWARA no Tabiko, her father was FUJIWARA no Momokawa who has rendered distinguished service for enthroning Emperor Kanmu, and the other was TABIHI no Takako who had hereditary title of Mahito kabane (the seventh Emperor Senka), both of them were considered as special cases. Consequently, the remaining possibility was Imperial Princess, although the title of Hi in the Heian period got lower, the advantage for an Imperial Prince whose mother was from Imperial family having prior right to succession to the Imperial throne since ancient time was valued for aiming at family succession to the throne. With the increase of succession by direct descent, the Imperial princess who was qualified to Hi was limited to Emperor's maternal half-sister and aunt. On the other hand, the presence of Hi was an obstacle to nobles like Fujiwara clan who were expecting to participate in politics as a maternal relative, and actually a daughter of FUJIWARA no Mototsune, FUJIWARA no Onshi was blocked to enter into the court because the Imperial Princess Ishi was Hi. With the death of Imperial Princess Ishi as start, it is considered that there was a close connection between abolition of Hi (some scholars pointed out possibility that Imperial Princess Sonshi who entered into the court for Emperor Enyu was Hi) and the development of the regency. After that, Hi was replaced in the late Kamakura period, but the powerful maternal relative influenced succession to the Imperial Throne and the status of Hi was the third rank after Empress and the second consort of an emperor.
After the position of Hi disappeared, a word 'Hi' was used as a title for the spouse of male imperial family and it is included in the modern Imperial House Act.