Koretsugu was a leading master of the Fukuoka Ishido school, which stands alongside the lineage as one of the twin pillars of swordmaking. He is said to have been born in 'ei 5 (1628) and was commonly known as Hansanbyo'e, and also as Ippei. In Meireki 1 (1655), by order of Lord Kuroda Mitsuyuki, he traveled to , where he studied the tradition (-) under Daijo Sakon Koreichi. After three years he returned to his home province and entered the service of the Kuroda family. Because Koretsugu's heir Toshitsugu predeceased him, his cousin Moritsugu succeeded to the main line. Koretsugu died in the third month of Tenna 1 (1681) at the age of fifty-three. Together with Moritsugu, he is regarded as the foremost craftsman representing the Fukuoka Ishido group.
Koretsugu's style reflects the formative influence of his teacher Sakon Koreichi while achieving a distinctly personal expression. His forging characteristically shows dense that flows overall, becoming strongly -inclined in the lower half of the blade, a hallmark by which his work is distinguished from true old production. Fine adheres, and or stands out prominently. His is a brilliant mixed with , , and pointed elements, with plentiful and creating a flamboyant and varied temper. The tends toward a tight formation () accompanied by , and fine , , , and enliven the interior of the temper. A distinctive tendency toward -gakari (reverse-slanting movement) in the is frequently observed, along with the unique forms popularly called "ika no atama" ("squid heads"). His tempering runs deep, and the can become so broad as to reach the . Whether working in the typical shallow- construction or producing dignified, large-scale hono-to (votive swords), his command of the form is assured and without lapse.
The consistently identify Koretsugu's work as excelling in the -inspired -ba for which the Fukuoka Ishido school is celebrated. His finest pieces exhibit a bold and expansive style in which the hardened edge above the becomes especially lively, vigorously incorporating and while and function throughout. Several of his designated works bear date inscriptions and cutting-test records, including gold-inlaid appraisals, that provide documentary evidence of considerable value for the study of the school. Among smiths, Koretsugu stands as a representative figure whose work fully demonstrates the characteristic highlights of the Fukuoka Ishido tradition.