Izu no Kami Masafusa was the son of the swordsmith Ujifusa, known by the personal name Hyoemon. He moved to Kagoshima in Satsuma Province, where, as the observes, "he exerted a major influence on Satsuma swordsmithing." The name Masafusa continued through five generations down to the Bakumatsu period, but it is the who is regarded as the fountainhead of the Satsuma tradition. Signed works by him are extremely few, and dated examples even scarcer; only a small number bearing Kan'ei and Shoho dates are known, placing his active period around the Genna to Kan'ei eras (c. 1615-1644).
The designated works reveal a smith of intense, -driven character. The earliest designated (, 11th Session) -- "regarded as precious as a reference work since olden times" -- displays an with flowing tendency and abundant , and a temper mixed with in which "the is coarse" and " runs through, and long appear here and there." The notes that "at first glance it resembles works such as those of Hida no Kami Ujifusa, yet shows an even more intense style of workmanship, with strongly expressed coarse ." A from before his receipt of the Izu no Kami title (, 32nd Session) shows a comparatively calm -based temper, yet the Satsuma hallmarks remain unmistakable: heavy , deep , and and intermingled with coarse .
These few surviving works confirm Masafusa's position as the originator of the -laden, vigorous Satsuma aesthetic that would define the province's smithing lineage for generations. "It may well be," the writes, "that here lies the fountainhead of Satsuma swordsmithing."