Early life

Yi Geon was born on October 28, 1909, at Sadong Palace in Gwaninbang, central Hanseong. He was the eldest, illegitimate, son of Yi Kang, Prince Uihwa with Lady Jeong of Sugwandang Hall. His mother was one of the many concubines his father had a relation with.

His birth name was Yong-gil (용길, 勇吉), but he changed his name to Geon (건, 鍵) on March 17, 1924.

According to the decree of Emperor Meiji promulgated at the time of the annexation of Korea in 1910, only eight people: Yi Hui, Gojong of Korea, Yi Cheok, Sunjong of Korea, Yun Jeung-sun, Empress Sunjeong, Yi Eun, King Yeongchin, Yi Kang, King Uichin, Kim Suk, Queen Uichin, Yi Jae-myeon, King Heungchin, and Queen Heungchin; were granted a new status as royal family members. The remaining members of the royal family were also granted a new status, but not grand as their relatives and it was not officially included in the decree.

He graduated from Gyeongseong Kindergarten in June 1916 and entered Ilchul Elementary School on April 6, 1917. That same year Yi Geon’s younger half-brother, Yi Woo, was adopted by Yi Jun-yong in 1917, followed by a few adoptions of his half siblings, and inherited the throne. Which created a situation where the younger brother was a part of the royal family but the older brother was not.

Life in Japan

He was brought to Japan in 1918, and entered Gakushūin Primary School.^[2] Another source says that in 1921, Yi Geon was sent to Japan under the pretext of studying abroad and entered the elementary school where he graduated from the school in March 1923.^[3]

Yi Geon was arbitrarily given the title of 'Confucius' (공자, 公子) after studying in Japan. He was soon officially recognized as a member of the royal family only after the "Wang Gongga Gwebeom" (왕공가궤범, 王公家軌範) was promulgated in 1926. He was given the honorific title ‘Your Majesty’ (전하, 殿下), which was only used for him.

In April of the same year, he entered the Army Youth School. According to Article 59 of the 《Wanggonggawebeom》 (王公家軌範), which was enacted and promulgated as Japanese Imperial Ordinance No. 17, on December 1, 1926, to regulate the imperial family of the Korean Empire after the annexation. It stated that it was mandatory for the king, crown prince, prince royal, and the duke to be commissioned as military attachés in the army or navy after reaching the age of 18.

Yi Geon entered the art department as a member of the 42nd class of the Japanese Military Academy.

His half-uncle, Yunghui of Korea, also died on April 25, 1926, as the last “emperor” of Korea. He was then enlisted to be Jongcheok jibsa (종척집사, 宗戚執事) and led the funeral ceremony. On May 5, he was permanently appointed the rank.

Prior to and During World War II

On June 12, 1930, his father, Yi Kang, went into seclusion and inherited the throne, becoming the new owner of Sadong Palace and was now being called ‘His Highness, the Duke’ (공 전하).

Yi Geon graduated the academy in July 1930.

He married Yoshiko Matsudaira, a maternal cousin of Princess Masako of Nashimoto and member of the Matsudaira clan, on October 5, 1931, in Tokyo.^[4] Yoshiko was a half-sister to Count Tadamitsu Hirohashi, a relative on her mother's side, and changed her name to Seiko Hirohashi. After she married Yi Geon, she was called Duchess Consort of Yi Geon, Yi Seiko (이건공비 이세이코, 李誠子).

Their marriage raises a few questions or curiosities due to the differences in personalities; Seiko was sociable, while Geon had a quiet personality. But they eventually had two sons and a daughter.

On October 25, he was commissioned in the Imperial Japanese Army as a second lieutenant of cavalry, and was assigned to the Guards Cavalry Regiment.

Yi Geon was promoted to Lieutenant in 1932 and graduated from the Army Cavalry School in July 1933 where he was later promoted to cavalry lieutenant in August that same year.

He was soon appointed as an instructor at the Military Academy on March 15, 1935, and promoted to Captain on August 1, 1936. That same year on Deception 17th, Yi Geon entered the Japanese Army University as its 51st student. On December 28, in connection with his admission to the University, Yi was transferred from being a horse riding instructor at the Military Academy to a member of the Guard Cavalry Regiment once again.

Yi later graduated from the University on December 8, 1938. From the university, besides Yi Geon, the only Koreans to have graduated there during the Japanese colonial period were Yi Eun, Yi Woo, and Hong Sa-ik. Two days later, he was appointed as company commander of the Guards Cavalry Regiment.

A year later, on September 13, 1939, he was transferred to the Military University Research Department as an instructor of horsemanship and military instructor for the Imperial Military Academy. He received further promotions to Major on August 1, 1940, and to Lieutenant-Colonel on March 1, 1943.

Unknowing that it would be last time, Yi Geon was again transferred to the Military University Research Department on May 15, 1944.

With the end of the Second World War in 1945, he concluded his military career with the rank of Colonel.

Post-war and later life

After World War II, he was not allowed to go back to Korea. After he lost royal status by order of the SCAP in October 1947, he became a commoner and was naturalized as a Japanese citizen in 1950. Then he changed his name to Kenichi Momoyama.^[5] This also led to his wife and children to changing their names. His wife Seiko changed her’s to Yoshiko and his children changed their’s to Tadahisa, Kinya, and Akiko.

In a blood test, Momoyama discovered that he was not the biological father of his eldest son, Yi Chung, to which he believed that Yoshiko committed adultery back in 1932 when he was out working under the Japanese army. During the war, the couple had a disagreement over Seiko's employment at a Ginza club.

Yi Geon suspected that her choice of employment would be trouble as Matsudaira worked at a Ginza club and was prone to be “too social” with the clients there. He eventually divorced Yoshiko in May 1951 and only claimed the custody of his second son, Yi Gi and daughter, Yi Ok-ja.

During his divorce process, Momoyama met Yoshiko Maeda (前田美子, Maeda Yoshiko)and married her in 1952 where they had a son, Momoyama Koya, that same year. His second wife later adopted his daughter and son, but his son was soon adopted again to become the adoptive son of Maeda’s father, Maeda Fujiyoshi (前田藤吉), to continue the Maeda clan.

Yi Geon and his family lived in a rented municipal house in Yono City, Saitama Prefecture. Before the war, he was famous as a sports car enthusiast and owned many vehicles, including the British sports car Alvis speed 20 and the American Pierce-Arrow large limousine. This led him to serve as president of the Classic Car Club of Japan (CCJC) from the 1970s until his death.

Years later, his second son Momoyama Koya (桃山孝哉) served as vice principal of Kaisei Gakuin School.

In December 21, 1990, he died in Japan. A funeral was held under the supervision of the Japanese imperial family and acquaintances. An old military classmate and Japanese imperial family member Prince Mikasa came all the way to Urawa City to attend the funeral.

After the death of Yi Gu, Yi Geon’s half-nephew, in 2005, who was the Crown Prince of Changdeok Palace, the King Yeongchin royal line was cut off, and the question of who would succeed him arose. According to the law of primogeniture succession, the heir of Yi Geon, the eldest son of King Uichin, King Yeongchin's older brother, is the heir through the natural lineage. However, in the case of Yi Geon, he was naturalized as a Japanese citizen and was now of mixed blood with Japan, so there could be problems with legitimacy.