Ko- (古手掻) is the founding and classical phase of the school, spanning the late period through the era. The school takes its name from the gate (転害門) of in Nara, near which its smiths lived and worked under the temple's patronage, and its founder is Kanenaga (包永), traditionally dated to around the Shōō era (c.1288-1293). Kanenaga is reckoned among the foremost Yamato masters: his signed are celebrated for combining the dignity of form with the disciplined grain of the Yamato tradition, and several are designated at the highest levels. The mainline continued through a second-generation Kanenaga and the founder's sons and pupils — Kanetsugu (包次), Kanekiyo (包清), and the early Kanezane (包眞) — into the period.
In workmanship Ko- is orthodox . The is with conspicuous , especially along the , thick with and threaded by . The is built on a narrow, -laden , frequently broken by and , with and along the and abundant and ; the runs to a or finish, often continuing into . Early works show a slender, deeply with a small , broadening into the more powerful forms of the period. The school's -rich grain and quiet, -active distinguish its blades from the -based exuberance of contemporaneous work and place squarely within the five Yamato schools.