Masaie is regarded as the founder of the Ko- school, the group of swordsmiths that flourished at in Bingo Province from the end of the period through the period. Bingo Province had been renowned since ancient times as a center for iron products, noted as early as Engi 14 (914) in the Seiji Yoryaku. Together with Masahiro, Masaie stands as one of the school's foremost representative figures, though it is thought that Masahiro may predate him chronologically. The name Masaie was borne by several generations, with signed works bearing dates of Bunnawa 2 (1353), Enbun 2 (1357), and Teiji 2 (1363) attesting to continuity through the era and likely into the period. Signature references record the titles Uemon no Jo and Saemon no Jo. Because the region contained many landed estates belonging to major temples and shrines of the Kinai heartland, such as Toji, Rengeoin, and Koyasan, it is conjectured that the pronounced Yamato temperament discernible in workmanship arose through exchanges with these central institutions. At the time, because certain works display an -like manner, influence from the smiths of neighboring is also considered.
The is characteristically mixed with , with the grain tending to stand prominently; toward the cutting edge, a flowing tendency incorporating is frequently observed. Fine adheres well and delicate enter, while the steel shows a tendency toward and a or faint -like appearance often stands out in the ground. The is based on , at times mixed with and shallow ; and appear, and adheres along a that characteristically tends toward or a slightly subdued quality, though examples with a bright and clear are also encountered. Along the , , , and uchi-noke are frequently intermingled, while fine and run through the tempered area. The is typically in tone, turning back in with , and is restrained in character. A critical distinction from Masahiro, whose works often show ordinary proportions with , is that Masaie is known for many examples with bold, imposing and powerfully built featuring wide , high , and broad construction that strongly reveals a Yamato coloration.
Across assessments, Masaie's works are recognized as overflowing with a distinctly Japanese coloration and manifesting Ko- characteristics to the fullest. The quality of both and is consistently noted as particularly superior even within the Ko- group, and the high level of forging skill is inferred from the absence of any looseness even in blades of exceptional length. Signed works by Masaie are extremely few, and dated examples rarer still, making those that survive precious source material for understanding the school's development. His bold, forceful constructions with ample and preservation are repeatedly praised, and his position as the school's founding figure, whose characteristic manner of working defined the Ko- identity at the intersection of Yamato discipline and Bingo provincial tradition, is firmly established in the designation record.