Yasutsugu (康次, also read Yasuji) was a founding figure of the Ko-Aoe school in Bitchu Province, traditionally held to have been a son of Moritsugu and active around the Kenryaku era (1211-1213). The Aoe school flourished along the lower course of the Takahashi River in the districts of Ko'i and Manju, a region long renowned as a source of iron. Together with Moritsugu, Sadatsugu, and Tsunetsugu, Yasutsugu ranks among the most representative smiths of the Ko-Aoe lineage. His most celebrated work is a long tachi transmitted as an heirloom of the Shimazu family, counted among the legendary Tenka Goken (Five Swords Under Heaven) alongside Dojigiri Yasutsuna and Odenta -- a blade of extraordinary flamboyance that stands unsurpassed among Ko-Aoe productions. Signed tachi by this smith are scarce but relatively numerous compared with other Ko-Aoe makers, and two examples reside in the Imperial Household collection.

Yasutsugu's forge work displays the hallmarks that define the Ko-Aoe identity. The jihada is built on itame intermixed with mokume, the grain standing conspicuously to produce the distinctive chirimen-hada -- the tightly compressed, crepe-like surface texture that is the school's most recognizable trait. Patches of jifu and ji-madara mottle the ground steel, while ji-nie adheres and fine chikei thread through the surface; on many examples a faint midare-utsuri rises in the ground. A dark-toned sumi-hada may also appear amid the standing grain. The hamon ranges from restrained suguha-based temper lines mixed with ko-midare and ko-choji to more vigorous compositions incorporating ko-gunome and scattered tobiyaki. In all modes the nioiguchi tends toward a shizumi (subdued) character with fine ko-nie well attached; ashi and yo enter freely, and kinsuji and sunagashi enliven the hardened edge. The boshi typically turns in ko-maru, sometimes with a midare-komi tendency. Tang construction follows school convention: o-sujikai yasurime file marks, deeply cut machi, and the signature placed on the haki-ura side in a characteristically fine chisel, positioned low and close to the mune -- a practice that distinguishes Aoe work from contemporary Ko-Bizen traditions.

Yasutsugu occupies a pivotal position in the study of Bitchu swordsmithing. His oeuvre demonstrates unusual breadth, encompassing both the quiet, austere aesthetic -- the shibui restraint -- that typifies Ko-Aoe production and, at its apex, a flamboyant midare-ba of extraordinary vitality that rivals anything produced in the Bizen tradition. Surviving ubu tachi exhibit the commanding sugata of their era: high koshizori, pronounced funbari, and an imposing bearing that conveys the dignified stature of early Kamakura-period long swords. The consistency of his technical vocabulary across signed, unsigned, and attributed works -- the standing grain, the crepe-textured surface, the subdued yet nie-rich temper -- provides scholars with a reliable framework for identifying Ko-Aoe productions more broadly. That examples of his work have been preserved in the Imperial collection and among the most storied feudal heirlooms attests to the enduring regard in which this smith has been held across centuries of Japanese sword connoisseurship.