Nagasone Okisato, who took the smithing name Kotetsu (虎徹), began his career not at the forge of the swordsmith but at that of the armorer, working as a katchūshi in Province. Around Meireki 2 (1656), when he was about fifty years of age, he relocated to and changed his profession to swordsmithing, settling in the area of Tōeizan and Shinobugaoka. The record that his common name was Sannojō; he first signed Okisato, and after taking Buddhist vows styled himself Kotetsu Nyūdō. The written form of the name itself moved through stages, beginning with the characters 古鉄 ("old iron"), shifting to 虎徹, and from the eighth month of 4 (1664) onward to the variant forms 乕徹 and 馬徹. His dated blades run from Meireki 2 to Enpō 5 (1677), the latter being the last year for which work survives. His pupil and adopted son Nagasone Okimasa (興正) succeeded him as the second generation, working from 13 (1673) to Genroku 3 (1690); the registers note that certain blades signed by the master were in fact Okimasa's , or substitute work.
The describe a shared vocabulary that holds across the line. The foundation is a strong , forged as a tightly packed in which adheres thickly, fine enter, and the steel reads bright and clear; a mottled inclusion the catalogues call teko-tetsu or appears as a recognized trait. The is the robust - silhouette, with shallow , marked taper from base to tip, and a . Most blades open with a short straight . The falls into two recorded phases: the earlier hyōtan-ba, a gourd pattern in which large and small connect with pronounced rises and falls, and the later , the rosary-bead temper of round-headed linked along an even line, often worked over a shallow . Thick enter, the runs deep, lies thickly, and and play within the , which is consistently bright. A that crosses the in is named in the registers as the characteristic "Kotetsu ." Several blades carry cut by the smith's own hand, from Fūjin and Raijin to and Sanskrit seed-syllables, and the cutting reputation is documented through gold-inlaid test inscriptions on the tang.
For the collector, recognition turns on these named points: the of the mature work, the bright order of and , the teko-tetsu in the ground, and the long signature in fine chisel-work centered on the , with the first character cut so as to overlap the . The catalogues distinguish the earlier manner of writing the tora character from the later -tora phase, which they treat as a dating tool. The cutting-test inscriptions give Kotetsu a place among the swords valued for performance as well as form, and the rarity of certain shapes adds weight: by his hand number perhaps fewer than ten, and in those the Sōshū-den feeling is described as especially strong. Named works carry provenance through the front rank of collecting, among them a transmitted in the Nabeshima family of and known as "Nabeshima Tetsu," confirmed by a of Satō Kanzan, and blades formerly held by figures such as Inukai Bokudō. Okimasa stands a step behind his master in the registers, his deeper and thicker read as a personal voice within the inherited . The line occupies a central position in the study of , its work measured by the strength of its steel and the discipline of its bright temper.