Mitsuyo (also read Kosei) was a smith of the Miike school lineage, originally rooted in Chikugo Province. By the Nanbokucho period, later generations of this line had dispersed to various provinces, and at least one branch relocated to Aki Province. A dated tanto bearing the inscription "Aki no Kuni ju" with a Meitoku 2 (1391) date provides direct documentary evidence of this migration. The NBTHK regards this relocation as a matter of considerable historical significance and describes such works as "extremely valuable material" for understanding the school's geographic diffusion. The dedicatory inscriptions appearing on certain Aki Province works -- naming figures such as Sanechika and Minechika -- remain unidentified, adding a layer of provenance interest to these pieces.

Mitsuyo's characteristic forging is itame-hada flowing toward masame-gakari with a standing grain tendency, enlivened by chikei and the presence of ji-nie. In more tightly forged examples, the ground becomes a dense ko-itame. The temper is consistently a narrow suguha or hoso-suguha in ko-nie-deki, with a slight hotsure tendency and minimal ashi; kinsuji enters within the hardened edge. The boshi characteristically shows hakikake turning to ko-maru. Blade forms range from hira-zukuri tanto with uchi-zori and somewhat thick kasane to broader, thinner pieces with shallow curvature consistent with Nanbokucho-period proportions. Carvings include bonji above koshi-hi, katana-hi with suken in relief, and bo-hi with soe-hi terminating in kaki-nagashi.

The NBTHK appraises Mitsuyo's workmanship as "superior" and judges it "on par with later Miike school work," situating these pieces squarely within the established technical and aesthetic standards of that lineage. Imperial provenance -- transmission through the Katsura-no-miya family -- attests to the esteem in which certain examples have been held. Within the broader Miike tradition, Mitsuyo represents the school's continued vitality during the Nanbokucho era and its capacity to maintain a refined, suguha-based idiom even as the lineage extended into new provincial workshops.