The Kievan Rus isn’t the same thing as Ukraine. Ukraine literally means borderlands, as in the borderlands of the Russian and Polish empires. Modern Ukrainian identity comes from the Cossacks who lived in these areas and had a high degree of political autonomy under the Russian Empire. There was no state called Ukraine until the Soviet Union.
Kievan Rus’ is the ancient predecessor. Ukraine is its most direct heir. They’re not the same, but Ukraine grew out of Kievan Rus’ ...far more than Russia did. So no, Ukraine isn’t just leftover Russia. If anything, Russia is a breakaway offshoot of Ukraine’s past ...not the other way around.