Yushi (privately adopted child) (猶子)
Yushi is a system that existed prior to the Meiji period to establish a parent-child relationship with a child who was not kin. Unlike adoption, however, Yushi was based on a contract and had a weak connection, and a child's surname was not changed, so it was rather a fictitious parent-child relationship (in which a parent became guardian in many cases).
It is said that the word Yushi originally meant a child of siblings in the ancient China. The oldest record of Yushi was allegedly the one of MINAMOTO no Sadamu who became the Yushi of Emperor Junna, but the difference between Yushi and Yoshi (adoption) is said to be not so clear until the late Heian period.
It was written in a Johyobun (memorial to the Emperor) dated November 18, 872 of FUJIWARA no Mototsune, who was then the Minister of the Right and a child of FURIWARA no Nagara, that he was the Yushi of his uncle, late FUJIWARA no Yoshifusa (former Sessho Daijo daijin [Regent and the Grand Minister])
But Yoshifusa did not have other son and the privilege of Oni (an automatic promotion system in which an Imperial title are conferred) and the family property were inherited by Mototsune, therefore today it is regarded as Yoshi (adoption), not Yushi. Furthermore, MINAMOTO no Morofusa (a child of Imperial Prince Tomohira) became the Yushi of FUJIWARA no Yorimichi, who was the husband of his older sister and Kanpaku (chief adviser to the Emperor) with his surname of Minamoto unchanged, but the record of that time described him as the "adopted child with different surname" ("Shoyuki"[the diary of FUJIWARA no Sanesuke]). Another case was TAIRA no Kiyokuni who was the child of FUJIWARA no Kunitsuna, became the Yushi of TAIRA no Kiyomori, and then changed his surname to TAIRA. In the Kamakura period, the difference between Yushi and an adopted child became clear and the practice was widely spread even among samurai and Buddhist monks.
(Though in later years there were some cases that a parent had no real child and Yushi became heir, they were regarded as exceptions.)
The objectives of Yushi were as the follows:
1. Advantages in promotion such as Kani (official court rank)
2. Advantages in marriage
3. Strengthening of relationship with other clan
The typical examples of 1. include Mansai who became the Yushi of Yoshimitsu ASHIKAGA, Prince Hikohito of Fushimi no miya (Emperor Gohanazono) who became the Yushi of the retired Emperor Gokomatsu in order to add dignity for his succession to the Imperial Throne, and Hideyoshi TOYOTOMI who became Kanpaku (chief adviser to the Emperor) as the Yushi of Sakihisa KONOE.
As the typical examples of 2., FUJIWARA no Moshi who became the Yushi of FUJIWARA no Yoshinobu and married with Emperor Gosanjo to give birth to Emperor Shirakawa, and TAIRA no Tokuko who became the Yushi of the Cloistered Emperor Goshirakawa and married with Emperor Takakura to give birth to Emperor Antoku are well-known.
The typical examples of 3. included Yoritsuna UTSUNOMIYA who became the Yushi of Masamitsu OYAMA and made an alliance with him, and Hideie UKITA who became the Yushi of Hideyoshi HASHIBA (TOYOTOMI) and succeeded the reigns of the UKITA clan under his guardian.
However, there have been few rare cases that Yushi relationship ended in tragedy. For example, FUJIWARA no Yorinaga who became the Yushi of his older brother FUJIWARA no Tadamichi fought with his brother at Hogen War, and Kugyo who became the Yushi of his uncle, MINAMOTO no Sanetomo, assassinated his uncle uncle.