The K˘saka family were from the southern end of the Kawanakajima area. They entered Shingen's service in 1553.
Masanobu, the son of the farmer Kasuga Osumi, is said to have entered the service of Takeda Shingen at the age of 16 (but this cannot be correct - at age 26?). He rapidly rose through the ranks to become one of Shingen's most valued councellors and generals. He and Shingen probably enjoyed a homosexual relationship (see letters adressed from Shingen to Masanobu). This would not have been uncommon for members of the bushi class.
The bulk of the evidence points at Masanobu as the main author of the Takeda family history K˘yo Gunkan (compiled by Obata Kagenori, who probably also wrote the sections on strategy). The work would have been continued after his death by his nephew Kasuga Sojiro before it was passed on to the Obata family. Authorship of the different sections of the K˘yo Gunkan is far from fully ascertained, however, and might never be.
Masanobu was made commander of Kaizu castle, which Shingen had built to control the Kawanakajima plain. He led the Saijosan "woodpecker force" at 4th Kawanakajima; his speedy return to the the battle on the plain after he had found Kenshin's abandoned camp saved the Takeda force.
There is evidence to suggest relations between Shingen's heir Katsuyori and Masanobu were strained. Masanobu was campaigning with his 450 personal horsemen and other Takeda troops against the Uesugi at the time of the Nagashino campaign. Alerted to the disastrous defeat he force-marched his army south to safeguard the retreat of the remains of Takeda Katsuyori's force. After having critizised him on several occasions he retired, to die of illness in 1578.
Kosaka Masanobu's uma jirushi (personal banner)
The sashimono (back banner) used by Kosaka Masanobu's followers was probably a smaller version of his uma jirushi.
Kosaka Masanobu as he might have looked leading the Saijosan "Woodpecker" force to the rescue at the 4th battle of Kawanakajima. Baba Nobuharus troops (black and white banners) and Obu Toramasas red-armoured "fire" troops were both under his command.
Kosaka Masanobu holding the army's flank.