Within the long span of Naminohira, Ko-Naminohira (古波平) marks the formative chapter, the body of smiths and works held in the to not descend past the period. The place the school's base at Naminohira in Taniyama District of Satsuma Province in southern Kyushu, where, by tradition, a Yamato smith named Masakuni settled in the late era around Eien (987 to 989) and founded the line. His son Yukiyasu stands at the head of the working lineage, and the name Yukiyasu passed through generation after generation as the main line (), with smiths favoring the characters yuki (行) and (安). The attach the lineage to the Yamato Senjuin tradition; one record fixes the earliest dated inscription in all of Japanese swords to a Heiji 1 (1159) read as Yukimasa, recorded in the as Ko-Hahei. Beyond Yukiyasu the corpus names Yasuji, Chikayasu, Yasumitsu, Tomoyasu, and Yamauchi Yasuyuki, the named members of this early generation.
The phase style sits close to Yamato yet keeps a Satsuma accent the return to repeatedly. The forging is with a flowing and tendency toward the edge, the grain at times standing (), carrying well-adhered and fine ; the steel is described as , a viscous, soft-feeling ground, often with a whitish . The temper is a narrow to , shading at times into or shallow , worked in with frequent , , , and intermittent and . Two diagnostics recur: the tends to , a moist, subdued quality, and the temper drops away at the base in . The is archaic, slender with deep and in the older , the commonly . The frame this against the later Sue-Naminohira only by exclusion: Ko-Naminohira is the early, hand-built work that does not descend beyond , set apart from the school's disorder and decline noted in one record, rather than the routinized later output.
For , the converge on a recognition cluster: ground, , base , and fine / within a of strong Yamato cast, an -like flavor noted in older works. The records caution that this conservative manner shows little change by period, so a missing nengō makes dating difficult and the form carries weight. Several align Ko-Naminohira with other Kyushu classical lines, naming Bungo Yukihira and Miike Mitsuyo as kin in archaic flavor. For provenance and benchmark, the texts cite the at Sanage Shrine as Yukiyasu's oldest, the named "Sasanuki" transmitted in the Kabayama family of the Shimazu, and a Kareki 2 (1327) dated , the rarity of signed and dated early pieces giving the surviving examples their documentary weight.