On a of his debut years Kotetsu cut his own biography into the : a native of who, past half a hundred years (半百), came to live in of and gave himself to the smith's craft. Nagasone Okisato, called Sannojo, who after taking lay orders signed Kotetsu Nyudo, was originally an armorer (katchu-shi) of ; around Meireki 2 (1656), when he was about fifty years old, he moved to and turned swordsmith, a conversion the published commentary restates on blade after blade. Dated work runs from that Meireki 2 to Enpo 5 (1677), the year before his death: one of the most celebrated careers in the Japanese sword fits inside two decades. The sources explain the sudden fame concretely: he brought from the armorer's forge a rare skill in treating iron, he excelled at carving, and he won his name by devising a strikingly novel , the (数珠刃という斬新な刃文を創意工夫したことから名声をはせた), the proven sharpness of the edge and the beauty of the carving (その利刃と彫物の美しさ) doing the rest. The modern commentary calls him the most popular smith of the entire age (新刀随一の人気工).
The standing formula of the published sources gives his work in one sentence: the is strong, and the and are bright and clear (彼の作風は地鉄が強く、地刃が明るく冴えるのが特色); most of his blades open with a (その作刀の多くに焼出しがあり), the short straight start of the temper above the . Above it, in the mature work, runs the that made the name: the , the peculiar (独得の互の目乱れ) whose round-headed (頭の丸い互の目) link so evenly that the crests stand almost in one line, like a string of prayer beads. Thick enter frequently, the is deep, lies thick, fine and run through the , and the is bright and clear. The is typically with a small round return, and in his later years he often burned a across the itself, a habit one text calls the idiosyncratic gesture of Kotetsu (乕徹独特の所作).
The is the armorer's legacy, and the sources say so directly. Of a of Meireki 2 or 3, his first manner, the commentary writes that the forging of the is good even among his own works, well kneaded and extremely strong (地鉄の鍛は同作中でもよく、よくねれて極めて強い); in those early blades a large open (大肌) surfaces toward the middle, and the so-called tekogane (テコ鉄), the armorer's billet iron, appears in the . In the prime the forging tightens to a knit to the utmost, thick and dust-fine, fine, the steel clear and bright, while flecks of (地斑) still surface here and there. The are his own hand: the early in splendid openwork (欄間透) is read straight from the carving of the Kinai school (記内彫), the add- moves from dosaku horinokore (同作彫之) to dosaku (彫物同作), the shinkitae- addendum (真鍛作) marks selected forgings, and even unsigned carvings are given to him outright on the fineness of the chisel.
The dates his blades almost by the signature alone. He signed first with the old-iron characters (古鉄), a form with almost no parallel; then in the manner; and from the eighth month of 4 (1664) in the -tora form, both occurring within that single month. The early period belongs to the hyotan-ba, shallow paired in twos (瓢箪刃と称される浅い互の目が二つずつ連れた刃) whose fused silhouettes suggest the double gourd, which one text dates as the often seen from the end of Manji into the first years of (万治の末年乃至寛文初年頃に多くみられる刃文). The later period carries the over the typical - (いわゆる寛文新刀の典型的な姿): tapering markedly from base to tip, shallow , a compact . The texts set the summit late, from 10 to about Enpo 2 or 3 (虎徹は寛文十年以後、延宝二、三年頃までがその大成期である), call the sober blades of about 8 mostly tight in the , short of flamboyance but solid (華麗さは乏しくとも堅実なものが多い), and praise a of Enpo 3 or 4 as a work of complete ripeness (全く円熟の作). Two registers stand aside. is comparatively rare for him, and when he took it up the as a rule tightens (匂口がしまりごころとなるのが通例). are rarer still, probably not reaching ten extant pieces (短刀の作は極めて少なく恐らく十口に及ばない); in them the stands out, the runs deep with coarse , and mingle, and the feeling of is strong, one text reading the aim as lying in the direction of Go (江辺).
The cutting tests are nearly a second signature. Many of his designated blades carry gold-inlaid attestations by Yamano Kaemon no Jo Nagahisa and Yamano Kanjuro Hisahide, the leading authorities on test-cutting of the day (当時試切の大家), recording two-body and three-body cuts (三ツ胴截断) dated to the day. These inscriptions sit on the beside his own as certifications of the edge, never as signatures. That outnumber , with frequent masterpieces among them, one text lays to commissions from wealthy merchants more than from the warrior class (武士の注文よりも富商の注文が多かったが為であろう). Honma Junji's verdict stands in the Bijutsuhin records: among the - smiths, Kotetsu of the east is, with Sukehiro and Shinkai of the west, beyond question of the first rank (東の虎徹と西の助広並びに真改とともに第一人者であることは無論である); the note adds that a market price overtaking the old masterpieces (その市価が古名刀を凌駕する) goes too far. The Nagasone forge outlived him: Okimasa, his adopted son, succeeded as the second-generation Kotetsu, with Okinao and Okihisa completing the gate, Okinao read as a hand during the master's lifetime.
Fujishiro rates him Sai-jo , the top grade. The official record carries 129 designated works: five Important Cultural Properties, ten and 102 , 112 in those two tiers together, and ten prewar Bijutsuhin. The record is almost entirely signed, 115 signed blades against a single unsigned piece, so the chronology above doubles as the connoisseur's map of the work. The recorded provenance runs through the great houses: the Nabeshima of , the Shimazu, the Okubo of Odawara, the Uwajima Date, the Maeda of and the Imperial Family; one blade passed through the house of Yamada Asaemon, the prewar premier Inukai Bokudo owned the wind-and-thunder-gods of Enpo 5, his last year, and a blade of 1 rests in the Tsukamoto Museum of Art. The five Important Cultural Properties are patrimony, preserved beyond the market; nearly everything else stands in the and registers, long held in private collections, so a genuine Kotetsu surfaces only rarely and commands the very top of the market when it does. For the connoisseur the recognition runs ahead of the : a over a strong, bright in a - reads as Kotetsu before the signature is ever examined.