Nagasone Kotetsu was originally an armor craftsman (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, and he first styled himself Okisato; after taking Buddhist vows he became known as "Kotetsu Nyudo." At the outset he employed the characters 古鉄 for "Kotetsu," later adopting the characters 虎徹, and from the eighth month of 4 (1664) onward he used the distinctive form 乕徹. This final change of characters marks the division between his earlier "" phase and his later "Hakotora" phase, a periodization central to the study of his oeuvre. Among his dated works, the earliest is Meireki 2 and the latest is Enpo 5 (1677). His works in the format are exceedingly few, likely not reaching even ten extant pieces, and certain rare signature configurations — such as the "-oki 'Nyudo wa Kotetsu' nine-character signature" — survive in only a handful of examples.
A defining feature of Kotetsu's workmanship is the strength of the and the bright, clear appearance of both and — a quality the consistently describes as akaruku . His forging is characteristically a tightly packed , upon which extremely fine adheres thickly and fine enter well; the steel is clear. Many of his blades display at the base. In his earlier period, he produced a with pronounced undulations in which large and small connect — the pattern termed hyotan-ba (gourd pattern) — carrying a Seki-influenced coloration. In his later Hakotora period, the variations in height become restrained, and he favored a distinctive linked with rounded heads, commonly called (rosary-bead pattern). His in this later phase reveals a characteristic practice of carrying the temper into the with and returning in with . Internal activities are rich throughout: , , , and appear in his finer works, and the is consistently bright and clear. In some pieces, particularly his rare , the feeling of - is notably strong, with conspicuous and coarse-grained . His blades characteristically possess ample -oki, yielding a heavy, substantial that speaks to robust construction.
Kotetsu's technique has long been highly esteemed, and the designations confirm this standing with striking consistency. His works are repeatedly described as pieces "in which Kotetsu's true strengths are fully manifested" and "his skill is displayed to its fullest," with individual blades singled out as a "masterwork" () or a "consummate work" (kaishin no ippo). The emphasize both and that are "bright and clear" as the essential hallmark by which his hand is recognized. Within the broader context of -, his present the archetypal silhouette of the era — standard or somewhat wide , noticeable taper from base to tip, shallow , and — yet it is the luminous clarity of the steel and the mastery of the temper that set his work apart. Whether in the bold, active manner of his transitional pieces or the calm refinement of his fully mature Hakotora works, Kotetsu stands as one of the supreme figures of the period, a smith whose workmanship the consistently judges to represent an "outstanding level" and "conspicuously fine" achievement.