Hiromitsu was a leading swordsmith of the Soshu tradition active during the Nanbokucho period (mid-fourteenth century), working in Sagami Province. He emerged as a direct successor to Sadamune within the lineage descending from Masamune, and together with his contemporary Akihiro he brought the Soshu school to its late-medieval apex. Dated works by Hiromitsu range from the Ko'o era through Bunna, Enbun, Ko'an, and Joji, establishing that he was senior to Akihiro, whose earliest dated pieces begin only at Enbun. Hiromitsu customarily signed with the long inscription "Sagami no Kuni junin Hiromitsu," and one rare tachi preserves the court title Saemon no Jo -- a detail of extraordinary documentary importance. Reliably signed tachi by the smith are exceedingly scarce; the overwhelming majority of his extant oeuvre consists of large-format tanto and hirazukuri ko-wakizashi that embody the bold, wide-bodied proportions characteristic of the Enbun-Joji period.

Hiromitsu's defining technical achievement is the perfection of the hitatsura (full-temper) style. While a tendency toward hitatsura can occasionally be discerned in Sadamune's work, it appears there as something incidental -- no more than scattered yubashiri (diffuse hardened patches). In Hiromitsu's hands, hitatsura became a fully realized and deliberately orchestrated mode of tempering. His hamon typically centers on choji-midare mixed with gunome, incorporating the distinctive round-headed clusters known as dango-choji. Thick nie (martensite crystals) adhere throughout; vigorous kinsuji and sunagashi streak through the tempered zone; and abundant tobiyaki and muneyaki unite to produce a flamboyant full-temper domain. The jigane displays itame mixed with mokume, often tending toward hada-dachi (standing grain), with dense ji-nie and plentiful chikei (dark lines of nie in the ground steel) -- hallmarks of Soshu forging at its most accomplished. The boshi characteristically enters in midare-komi and rises to a pointed or flame-like (kaen) configuration with deep turn-back, frequently exhibiting hakikake. On rarer occasions, Hiromitsu produced works in suguha (straight temper), demonstrating a broader technical range than is commonly recognized.

Hiromitsu's significance within the history of Japanese swordsmithing is profound. By bringing hitatsura to full maturity, he and Akihiro established a tempering paradigm that exerted lasting influence not only on later Soshu smiths but on swordsmiths across multiple provinces. His works were transmitted in some of the most distinguished collections of the Edo period, including the Kishu Tokugawa, Echizen Matsudaira, Bizen Ikeda, and Nanbu families. Hon'ami appraisals attest to the esteem in which his blades were held, with valuations recorded at seven hundred kan. Many of his finest pieces survive in kenzen (sound and well-preserved) condition, their steel surfaces retaining the brilliant nioiguchi and dense crystalline activity that connect them, as NBTHK examiners have noted, to an unbroken thread leading back to Masamune himself.