The kosode or "small sleeve" refers to the small opening of the sleeve which differentiates it from the big open sleeves of the layers of Heian (794-1172 CE) court robes worn over it.
The kosode was originally an undergarment which came out from under the karaginu-mo during the Kamakura period (1172-1333 CE) and transitioned to outerwear.
Many of the characteristics of what Westerners associate with traditional Japanese style developed after 1600.
I also recommend hunting down Money Hickman's Japan's Golden Age: Momoyama at the library or bookstore as it contains a number of examples in full color, as well as portraits of men and women from the period.My own first attempts at Japanese clothing were interpretations of Kamakura period clothing using information from the "Reconstructing History" site, which unfortunately is no longer posted there.Also invaluable is the Costume Museum in Kyoto at where you can browse pictures of their collection by historical period as well as a textile gallery which will help you in your quest for fabric that looks right." Occasionally, they will test the waters with a Trial Balloon Question.A more cynical subtrope is when the person is actually the Reverse Mole, but the temptation of The Dark Side gradually causes them to discard their initial good cause and become everything they originally despised.
It was more a matter of learning about stylistic differences of a later period.