Izu no Kami Masafusa was the son of the Mino swordsmith Ujifusa, known by the personal name Hyoemon. He moved to Kagoshima in Satsuma Province, where, as the NBTHK 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 shodai who is regarded as the fountainhead of the Satsuma shinto 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, nie-driven character. The earliest designated katana (Juyo, 11th Session) -- "regarded as precious as a reference work since olden times" -- displays an itame-hada with flowing tendency and abundant chikei, and a ko-notare temper mixed with gunome in which "the nie is coarse" and "sunagashi runs through, and long kinsuji appear here and there." The setsumei 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 nie." A wakizashi from before his receipt of the Izu no Kami title (Juyo, 32nd Session) shows a comparatively calm suguha-based temper, yet the Satsuma hallmarks remain unmistakable: heavy ji-nie, deep nioi, and kinsuji and sunagashi intermingled with coarse nie.

These few surviving works confirm Masafusa's position as the originator of the nie-laden, vigorous Satsuma aesthetic that would define the province's smithing lineage for generations. "It may well be," the NBTHK writes, "that here lies the fountainhead of Satsuma swordsmithing."