Muramasa is the founder of the Sengo line of Kuwana and the smith who set the manner that bears his name, and the published sources fix him firmly to a single point in time. The oldest extant date in the whole line is Bunki 1 (1501), carried by the dated signed "Seshu Kuwana-ju Uemon-no-jo Fujiwara Muramasa " preserved as a blade of the 40th session; with the full signature giving the residence, the personal name Uemon-no-jo and the smith name, the published commentary calls it "of high documentary value for research on Muramasa" (同工研究上の資料価値も高い). From that fixed point the prevailing scheme reads the Bunki pieces as the first generation, the Tenbun as the second, the Tensho as the third. Against this dated record the institution sets aside the famous legend: Muramasa, it writes, "was popularly transmitted as a pupil of Masamune, but this is unfounded" (俗に正宗の弟子と伝えられたが不当), a smith whose line begins instead "near the close of the period" (室町末期近くにはじまる刀工). His manner shares common features with , Shimada and Sue-, and resembles Heianjo Nagayoshi above all, so closely that the sources raise the view the two stood in a teacher-pupil relation: "it is especially close to that of Heianjo Nagayoshi" (平安城長吉とは特に似る).
The feature the published commentary names again and again as his own is the symmetry of his temper. "The characteristic of his manner," one description states, "is that the on and match, and the valleys of the press in toward the edge" (彼の作風の特色は表裏の刃文が揃い、乱の谷が刃先にせまることである). The mirror-image alignment of front and back is the Muramasa hallmark, and the sources return to it on blade after blade: a is called typical because "the hatori on both front and back is precisely matched, clearly demonstrating a characteristic feature of Muramasa." Over a body usually wide, often short with deep and , or a , he fires a -based mixed with and large , entering, the deep and thick adhering, with and along the edge. Box-shaped elements appear among the , and the valleys drive down sharply toward the . The runs straight to a small round point or enters the and sweeps; on the bolder pieces the temper carries down into .
The forging is that tightens then stands somewhat, with flowing grain mixed in and ; the published sources call it tending toward , frequent among works of the Sengo line, sometimes a tendency, the steel at times slightly blackish with a cast. On the finest pieces the is well refined and the bright, the thick and abundant. His general run of work, by contrast, is described more plainly, the "tight and subdued, with mura- commonly forming" (匂口は締りごころで沈み, 叢沸のつくのが一般である). What he is not is a smith. Even his most flamboyant , with conspicuous , is described only as "presenting an appearance akin to " rather than the full -strewn fields of the hands; the and the boxed are his, but the all-over temper is not, and this absence is itself a tell that separates him from the Hiromitsu-Akihiro manner the legend invoked.
The founder's hand divides between two registers of signature and one devotional group. The full long signature, "Seshu Kuwana-ju Uemon-no-jo Fujiwara Muramasa ," carries the dated Bunki blades and is read as the first generation; the bare two-character "Muramasa" is undated, its generation read from the chiselling and workmanship. On the dated Bunki 1 the temper is a with worked in a -dominant manner, carrying a tendency toward , which the published sources say differs somewhat in feeling from the work attributed to the second generation. Distinct from these is a group marked by Nichiren devotion. On the Bijutsuhin dated Eisho 10 (1513) the temper forms hakomidare low in the blade and runs up to , and matched, with the Lotus Sutra title Myoho--kyo cut on the blade itself; another carries with lotus, and an incised in superimposed relief. The generational boundary in these later-dated pieces is not yet firmly settled: the published commentary repeatedly notes that the divisions among the generations have not yet been firmly established, so the Eisho and Tenbun assignments are read with caution.
Where he stands among his neighbours the sources draw by his own traits rather than by borrowed comparison. His manner shares common features with , Shimada and Sue-; it is nearest Heianjo Nagayoshi; and the matched temper with the down-pressing valleys is the thread that runs through the whole line and marks it off. His boldest work reaches higher still. On the of the 16th session the temper is exceptionally wide, with large , the extremely deep and thick adhering, the deeply tempered to an appearance, and the judges read its model as the old master Go: it "may be thought to have taken as its model the manner of the old Go" (蓋し古作の江に倣ったものかと想われる). They call it a masterpiece of the first generation, one that "in this work in particular gives the impression of surpassing even the second generation" (二代村正をも上回る感があり). The line continued under the name through the second generation of the Tenbun years, held the most skilled and most prolific, and the third of the Tensho years, and from the school issued the pupils Masashige and Masazane. Its end was political rather than artistic: by the early modern period the name "came to be shunned by the Tokugawa house, and so the name could no longer be carried on" (その名跡は絶えている).
Muramasa is Sai-jo in Fujishiro's grading and rates 800 in the Toko Taikan, the representative swordsmith of late- . His Nichiren faith is read straight off the iron: because there are blades on which he "cut the title Namu-Myoho--kyo" (南無妙法蓮華経と題目を切りつけたもの) he is taken for a believer of the sect, and the published commentary ties this to the region, noting that "in the district of that time devotion to the sect was by no means uncommon" (当時伊勢地方に於ても、同宗の信仰が少くない). The unsigned of the 12th session, whose Muramasa signature was shaved away, is read by the published commentary as the work of a Nichiren believer forged for one such cleric. On record stand fourteen blades across the and tiers, one and thirteen , together with the Bijutsuhin Lotus Sutra ; that blade bears "Nabenobu" in silver inlay on the and is held to have been owned by Nabeshima Shinano-no-kami Katsushige, recorded at certification in the Nabeshima house. A first-generation survives "transmitted as a personal possession of Oda Nobunaga" (織田信長の指料と伝え), its mounting bearing the Oda crest in . None of the founder's blades carries the highest patrimonial designations, so none is locked permanently out of private hands; what survives sits almost wholly in the and tiers, held more often than traded. A first-generation Muramasa of secure date and matched temper is a landmark when it appears, and it appears only from time to time.