Aisu Ikosai, founder of Aisu Kage-no-Ryu, was born into the prominent bushi ("warrior") Aisu family in 1452, and lived from the middle to the late Muromachi Period (1392-1573). This was one of Japan's most turbulent periods. Ikosai's original name was Aisu Tarozaemon Hisatada, but he later took the name, 'Ikosai.' The Aisu family was a branch of the Kii clan of Kumano in the province of Kii, a powerful family in the center of the Kumano Bay area. They had been put in command of five castle areas by Shogun Morinaga Shinno (1308-1335) in the Nanbokucho (Southern Dynasty) Period.

Aisu Ikosai died in 1538 at the advanced age of 87.

It is not clear where or from whom Ikosai originially learned heiho (martial ways), but he was living during a period of great activity in fighting arts. Iizasa Ienao's Tenshin Shoden Katori Shinto Ryu was active in the Kanto area, and Chujo Hyogozu Nagahide had been spreading his Chujo Ryu in Mikawa-guni (present day Aichi-ken) more than a hundred years earlier. As well, it is thought that in the Kyoto-Nara area, a core group of Nen Ryu of disciples known as the "Six Men of the Capital" were spreading their art at the beginning of the 15th century.

Today, there remains virtually no contemporaneous material on Kage-no-Ryu. There is one important piece though, a reference to Kage-no-Ryu in a collection of Chinese martial essays called the Bubishi ("Records of Military Preparation"). The Bubishi was the work of the Ming Dynasty Chinese military writer, Bo Gengi (Ch. Mau Yuanyi). In a total of 240 volumes, he expounded on important points of military preparation. In volume no. 86, parts of the Kage-no-Ryu syllabus-accompanied with images of monkeys-are found:

Kage Ryu no mokuroku ( "The Kage Ryu Syllabus")
Enpi ("Monkey Flying")
Koryu ("Tiger-dragon)
Seigan ("Blue Shore")
Inken ("Shadow Vision")
Enkai ("Monkey Twirling")
Dai-san Yamakage ("Number 3 Mountain Shadow")

It is likely that Kage-no-Ryu had gained attention among Chinese military authorities from Japanese warriors and pirates who raided China's coasts during that period. Obviously, the military authorities were impressed with the swordsmanship of these Japanese fighters. The story of Ikosai's ken-no-satori (sword enlightenment) is that in the early 1500's, Ikosai, who already had been refining his sword skills for many years, made a visit to the temple of Uto Gongen. There he prayed for purification and enlightenment. A monkey-shaped god appeared in a dream and revealed the secrets.

Deciding to reveal this knowledge to selected students, Aisu Ikosai named his style "Kage-no-Ryu" ("Shadow Ryu"). Later, Ikosai's student, the renowned warrior and swordsman, Kamiizumi Ise-no-kami Hidetsuna developed his Shinkage Ryu ("New Shadow Ryu") based on the instruction he received from Ikosai. Shinkage Ryu thus includes the techniques Enpi, Enkai, Yamakage, etc. from Kage-no-Ryu.