Ippei Yasuyo, common name Tamaoki Koichi, was the eldest son of the Satsuma smith Ippei Yasusada, born in Enpo 8 (1680). The published sources record that he learned forging first under his father and then under Yamato no Kami Yasukuni of the main Naminohira line, and that in the first month of Kyoho 6 (1721) he was summoned together with Masakiyo by the eighth shogun Tokugawa Yoshimune to forge swords in Edo. In recognition of his skill the bakufu permitted him to cut the single-leaf hollyhock crest into his tang, and on his return journey the court appointed him Shume no Suke. He died in Kyoho 13 (1728) at the age of forty-nine, so that the great part of his recorded work falls into the narrow span of years between the Edo summons and that early death. The institutional verdict on him is consistent and almost formulaic across his designations: he and Masakiyo are 'the twin pillars of Satsuma shinto' (正清と並んで薩摩新刀の双璧).

That pairing carries within it the contrast by which his hand is recognized. Where Masakiyo favored a varied, Shizu-like midareba that mixes notare with gunome and pointed togariba, the published sources record that 'Yasuyo more often tempered a calm, notare-toned suguha' (安代は穏やかなのたれ調の直刃を多く焼いている). His characteristic temper is a suguha-cho, or a wide suguha, that undulates in shallow notare, deep in nioi, the nie laid on thick and strong with coarse ara-nie mixed through it. The feature the judges return to is what runs inside that quiet line: vigorous sunagashi together with long nie-suji and kinsuji, the streaming nie-lines the sources name 'the so-called imozuru of Satsuma' (いわゆる薩摩の芋蔓と称される沸筋), the yam-vine of the province. On his finest blades these run especially long and are called splendid, the activity in both ji and ha richer than usual.

Beneath that temper the jigane is a tightly packed ko-itame, at times mixed with ko-mokume or flowing toward nagare, with thick ji-nie and chikei entering, and a steel that takes on a markedly blackish tone. Against that dark jigane the bright nioiguchi and coarse nie of the edge stand out, which is much of why a Yasuyo blade reads as it does. The boshi is essentially straight and turns back in an o-maru or a ko-maru, the point swept with hakikake, on the deeper-tempered pieces vigorous enough to take a flame-like cast. His sugata is the other half of the impression: broad in the mihaba, thick in the kasane with ample hiraniku and a chu-kissaki, heavy in the hand and frankly imposing, and almost always preserved ubu with a deep kurijiri, higaki file marks and a long signature.

Within this one settled manner the published sources draw two finer registers. A small number of his blades lean toward masame, the itame flowing and the hada raised a little, and on one such Juyo katana the judges read the whole as 'workmanship that recalls the Yamato Hosho tradition' (大和保昌伝を見るような出来) and name it a representative piece; the Naminohira and Yamato roots of his training surface there in the standing grain. At the other end of his career, an early katana cut with a four-character signature below the peg-hole is identified as a formative work, and the sources note that such early pieces are comparatively few and valuable for understanding how his style began. The continuity matters more than the divisions: the nie-deki suguha is constant, read once over a calm ko-itame and once over a more standing, masame-tinged jigane.

What sets him apart is exactly what the judges name, and it is best read through his own work rather than against his neighbor's. His is the quiet suguha of the Satsuma pair, its strength carried not in the height of the temper but in the depth of its nioi, the thickness of its nie, and the long imozuru threads that course through it; his ji is dark and well forged, his sugata broad and robust. Of one Tokubetsu Juyo katana the published commentary writes that it is 'his masterpiece, showing outstanding workmanship among his works' (同作中抜群な出来映えを示した彼の傑作), the kind of judgment the sources reserve for the pieces in which the deep nioiguchi, the thick nie and the running kinsuji all come together on a grand, sound body.

For the collector Yasuyo is one of the most sought names in Satsuma shinto, and his record is almost wholly a signed one, twenty of his designated blades carrying his own inscription. Fujishiro grades him Jo-jo saku. He has no National Treasures and no Important Cultural Properties; his standing rests instead on four pieces at the Tokubetsu Juyo rank and thirteen at Juyo, with a further prewar Juyo Bijutsuhin, seventeen blades in the Tokubetsu Juyo and Juyo tiers in all. The provenance is unusually grand for a shinto smith: the sources record blades presented from the Shimazu house upward, one katana presented by Shimazu Tsugutoyo to the shogun Yoshimune, another 'presented by the Shimazu family to Konoe Sadaijin Iehisa' (島津家より近衛左大臣家久に献上), whose household prized it so highly that it sent Yasuyo silver and a set of poems in return. Imperial, Shimazu, Konoe and Tokugawa names run through his denrai. Almost none of this trades; the designated blades are held in long-established collections, and a signed Ippei Yasuyo comes to a private collector only seldom, and as one of the more rewarding Satsuma encounters when it does.