The Ko- school (Ko-) of Bingo Province emerged during the late period and flourished through the era, spanning roughly from the late thirteenth through the mid-fourteenth century. Bingo was strategically positioned as a corridor province between the capital region and western Japan, and in the and eras it contained numerous landed estates (shōen) held by major Kinai temples and shrines, including Tō- and Rengeō-in (Sanjūsangendō). This sustained contact with the Yamato heartland profoundly shaped the Ko- aesthetic, instilling what contemporary appraisers recognize as a pronounced Yamato character (Yamato kishitsu) that distinguishes the school from both its regional contemporaries and later generations.
The collective style of Ko- displays a refined synthesis of Yamato tradition adapted to Bingo sensibilities. The forging is characteristically mixed with , frequently showing a flowing tendency () that inclines toward near the cutting edge; the grain tends to stand out (), and a whitish cast () emerges in the steel, accompanied by fine and delicate . The temper line is typically —often narrow or medium in width—with the assuming a tight, subdued () character and densely adhering ; along the appear fine activities such as , uchi-noke, and occasional -like features. The is straight, turning back in a small rounded manner () with a long return—a configuration known as "'s taki-otoshi"—and often displays vigorous at the tip. In comparison with canonical Yamato work, Ko- exhibits generally weaker in both and , a gentler , and a more restrained ; these distinctions arise not from technical limitation but from a deliberate aesthetic that favors elegant restraint and quiet strength.
Blades attributed to Ko- embody the robust martial ideals of the period while preserving the dignified, unhurried elegance of Yamato tradition. The school's works typically present powerful, commanding shapes with wide , deep or moderate curvature, thick , and high ; many blades form extended configurations reflective of the period's battlefield demands. The fine activities that appear along the —, , , and nuanced marginal patterns—impart depth and resonance to the temper, while the understated and gentle convey archaic charm and serene integrity. The school's influence extended well into the period, as later generations continued to draw upon Ko-'s synthesis of Yamato refinement and Bingo martial vigor, securing the lineage's reputation as one of the most important regional traditions of the medieval era.