Nagasone Okisato, known as Kotetsu Nyudo, was originally an armor-maker (katchushi) in Province. Around Meireki 2 (1656), when he was approximately fifty years of age, he went to and changed his profession to swordsmithing. He is said to have used the common name Sannojo. He took the name Okisato, and after taking Buddhist vows he styled himself "Kotetsu Nyudo." His signature evolved in a well-documented progression: at first he used the characters 古鉄 (Kotetsu, "old iron"), later adopted the characters 虎徹, and from the eighth month of 4 (1664) onward he employed the characters 馬徹 (Batetsu). He also used variant characters such as 庸徹 and 備徹 at certain points in his career. Among his dated works, Meireki 2 represents the earliest, and Enpo 5 (1677) the latest. His training as an armorer left a lasting imprint on his craft, particularly evident in his blade carvings (), which display a strength and chisel-handling reminiscent of carving traditions.
Kotetsu's workmanship is characterized by a powerful and by both and appearing bright and clear (). Many of his blades show at the base of the temper. His career divides into two recognized phases. In his earlier period, the so-called "" phase, he tempered mixed with showing pronounced rises and falls, incorporating a pattern called hyotan-ba in which large and small connect in paired groupings; the style suggests an ideal modeled upon the tradition. In his later period, the "Hakotora" phase, the undulations of the temper show less variation, and he produced a distinctive linked with round-topped heads running in continuous succession, known as . The forging is typically a tightly packed with extremely fine adhering thickly and fine entering well. The is bright and clear, with deep and thick; and appear throughout, and in some works -like effects mingle along the temper line. His in the later period is distinctive, often tempered through the and turning back in . He also excelled at blade carvings, frequently adding his own with the supplementary inscription dosaku horu kore ("carved by the maker"); these are powerful and splendid, further enhancing the presence of his blades. Some works bear the additional inscription ("truly forged"), and many carry gold-inlaid cutting-test inscriptions by the renowned Yamano Ka'emon Nagahisa and Yamano Kanjuro Hisahide, attesting to their keen cutting ability.
Kotetsu's technical skill has long been held in the highest regard. His blades range from displaying the quintessential -era form -- standard width with evident taper, shallow , and -- to imposing of wide with thick and commanding . His and works are extremely scarce among extant examples and hold exceptional value as documentary material. Whether in the vigorous hyotan-ba manner of his early career or the masterful of his maturity, his works convey a bright clarity of steel, abundant internal activity, and a bold, spirited character that is unmistakably his own. The finest examples among his productions are described as blades of which Kotetsu could be proud -- exceptional performances in which both and are (sound and well-preserved), brimming with force and commanding presence.