Ninkan (birth date unknown - 1114) was the founder of the Tachikawa School (Esoteric Buddhism). He later changed his name to Rennen.
Ninkan was born as the eldest son of Sadaijin (Minister of the Left) MINAMOTO no Toshifusa of the Murakami-Genji (Minamoto clan). His uncle MINAMOTO no Akifusa held the position of Udaijin (Minister of the Right) and was progenitor of the Koga-Genji (Minamoto clan), his aunt FUJIWARA no Kenshi (Kataiko) was the consort of Emperor Shirakawa and the mother of Emperor Horikawa. His elder brother Shogaku was the founder of the Daigo Sanbo-in School, the largest of the six most prominent schools (Six Schools of Ono) of Shingon Sect Shugendo (mountain asceticism).
Ninkan enthusiastically studied the teachings of the Shingon Sect under Shokaku and ascended to the highest Shingon Sect rank of Ajari. He was also appointed gojiso (personal Buddhist monk) of Imperial Prince Sukehito (third son of Emperor Gosanjo) who supported the Murakami-Genji.
After Emperor Gosanjo abdicated the throne in favor of his first son, Imperial Prince Sadahito (who later became Emperor Shirakawa) in 1073, he expressed that he wished Imperial Prince Sukehito to be made emperor following his death. However, the then enthroned Emperor Shirakawa violated this will and installed his own son Imperial Prince Taruhito (who later became Emperor Horikawa) as emperor while himself exerting cloistered rule. Following the premature death of Emperor Horikawa, the then 5 year old Imperial Prince Munehito (who later became Emperor Toba) was enthroned. This exclusion of Imperial Prince Sukehito caused the Murakami-Genji to become increasingly dissatisfied.
In 1113, Ninkan, who believed that Imperial Prince Sukehito should be made emperor, planned to assassinate Emperor Toba. However, the plot was discovered before it could be carried out. The testimony boy named Senjumaru, whose role it was to carry out the assassination, led to the Ninkan's arrest and confession following questioning. In November of the same year, Nikkan was exiled to Ohito in Izu (Senjumaru Incident).
After being exiled to Izu, Ninkan changed his name to Rennen and worked to spread Shingon teachings.
It was during this period that he met Onmyoji (practitioner of Yin and Yang divination) named Kenren from Tachikawa, Musashi Province,
In addition to Kenren, he took other disciples including Kanren, Jakujo and Kansho who he instructed in the secrets of the Daigo Sanbo-in School.
In March 1114, he committed suicide by throwing himself from the peak of Mt. Jo. It is believed that his teachings were disseminated by his disciples to form the Tachikawa School.
Details remain unclear as the majority of the documents relating to the Tachikawa School were lost during the subsequent suppression of the school. This is also true of details pertaining to Ninkan, and there remain points of uncertainty in description of his life given above. Refer to the article entitled 'Tachikawa School (Esoteric Buddhism)' for more details.