He is also well known as Kamiizumi Isenokami (上泉 伊勢守) which was his name as a samurai official (武家官位, Bukekani) for a period of time.

[1] In his youth, he went to nearby Shimōsa Province (modern day Chiba Prefecture) and began studying Nen-ryū, and Shintō-ryū.

[1][4] In Yagyū Shinkage-ryū, it is believed that Kamiizumi learned Kage-ryū from Ikōsai, receiving full transmission around the age of 23.

[1] A few years later, Kamiizumi learned battle strategy and divination from a man named Ogasawara Ujitaka.

This same year, Uesugi Kenshin invaded western Kōzuke in order to re-take Hirai Castle.

[1] Kenshin seized total control of Kōzuke, installed Nagano Narimasa as governor in Minowa Castle, and returned to his base in Echigo.

[1] Shingen eventually found out, and in 1563 (some sources suggest 1566), he invaded western Kōzuke with a force of over 10,000 Kai soldiers.

One story is that he returned because of many friends still at the castle, and there he joined the service of Naitō Shūri-no-kami, the governor appointed by Shingen.

[2] Or, another story is that, impressed by Kamiizumi's valor, Shingen invited him to join the Takeda side after Minowa Castle fell.

Yoshiteru wrote a certificate of admiration, declaring Kamiizumi's school unparalleled in the land, and praising Marume's performance.

In 1572 he visited Yagyū Village, and then traveled back to eastern Japan, with a letter of introduction from Yamashina to the Yuki clan in Shimōsa Province.

Schools still extant today that claim Kamiizumi as a founder, co-founder, or progenitor include (Yagyū) Shinkage-ryū,[13] Jikishinkage-ryū,[14] Kashima Shin-ryū,[15] Taisha-ryū,[16] and Komagawa Kaishin-ryū.

The leather sleeve is made of cow or horse hide, but after being lacquered Kamakura Red, it resembles the skin of a hikigaeru.

The Honchō Bugei Shōden, a mid-Edo period collection of historical stories, related one of Kamiizumi travelling to Myōkōji Temple, in Ichinomiya, Aichi Prefecture.

Kamiizumi shaved his head, borrowed a kesa from a priest, and approached the barn with two rice balls.

Kamiizumi used the rice balls to lull the man into dropping his guard, and then quickly seized him and saved the child.