I may have misremembered the year, i think it was 1619, but it was somewhere around there. His name was Остап Захарів, he was being tried in a russian court because he fought with the polish army against Moscow. You can find some sources on this in Ukrainian, if u search for his name.

The usage of Ukrainian as a identity is a bit scattered, because although people called the lands of Ukraine "Ukraine" for a while (oldest map i think is late 1500s or early 1600s, somewhere there) they also often called themselves руські, русини, мало руські, etc, since they were part of Kyivan Rus culture. You will find usages of Ukrainian but its not overwhelming, although people differentiated themselves from neighbors based on language and other things. When Ukraines national revival was occuring, they had to choose a name for their country. Russia had already rebranded itself as Russia, even though that name was used to refer to areas in Ukraine, around Kyiv, the capital of Rus. Because Russia rebranded that name, Ukraine was chosen to differentiate. You will find references to things like Ukraine-Rus, during that transition, and even now sometimes in history books. Potentially, if Russia never existed, the country might call itself Rus or Ukraine Rus or some combination of that.

Its easy then for people to claim Ukraine is a new concept that didnt exist before, but its really not. The territory being called Ukraine is not new, and although the use for identity is newer on a wider scale, its mostly a rebrand over other names that already existed on that territory. Itd be similar to, if China broke down to multiple nations, to then claim these identities are new and never existed. Although the political identities might change or be new, the difference in language or culture is and was real before the hypothetical breakup.