Genkō Etan served as a bushō and monk during the Sengoku period.

Etan was an older brother of Imagawa Yoshimoto but outside the line of family succession.  There is a theory that he adopted the name of Imagawa Nagazane.

In 1517, Etan was born as an illegitimate son of Imagawa Ujichika.  Similar to Sengaku Shōhō (his younger brother of a different mother later known as Imagawa Yoshimoto) and a monk named Shōji Senjō, Etan entered the priesthood at an early age, becoming the chief priest of the Henjōkō Temple.

Previously, Imagawa Hikogorō was regarded as the second son of Ujichika, but, based on genealogy of the Imagawa in a compilation prepared prior to 1551, Genkō Etan is identified as the second son, confirming that Etan was the second son and illegitimate older brother of Hikogorō.

In 1536, Imagawa Ujiteru (the head of the Imagawa family) and his second younger brother, Imagawa Hikogorō, suddenly died one after another.  Over matters of succession, Etan was backed by the Fukushima clan from a base at Hanakura Castle, but was attacked by members of a faction in support of Sengaku Shōhō whereupon Etan killed himself at the Fumon Temple in Setoya.

Legend

Just prior to the Battle of Okehazama, Etan appeared in one of Yoshimoto’s dreams, warning him not to proceed with the deployment.  Yoshimoto responded that [the Oda] were his enemy so he would not listen, whereupon Etan responded that this is not about his feelings toward friends or enemies, but concern over decimation of the Imagawa clan, whereupon Yoshimoto awakened from the dream.  Yoshimoto deployed from Sunpu, but saw an image of Etan among the wisteria trees and placed his hand on the hilt of his sword.