I wanted to know about the origins of the people of Kiev from the late 9th to the mid-13th century, and why it is known as "Kievan Rus" if today its population is Ukrainian. Why is that, and where did they come from if they are not Russian, and how did they adopt the name Rus?
Depends on who you ask.
That is the short version, but it is a lot more complicated than that and delves into recent politics, Soviet politics, and then add hundreds of years and various Empires.
Perhaps the best way to describe it would be that Kievan Rus' is the ancestor of both Russian, Belorussian, and Ukrainian ethnicity's. All three share the same "root" linguistic language of "Eastern Slav," but that is a question better for /u/rusoved than me.
I think the better start would be to say that Russian and Ukrainian identities and by definition their ethnicity's did not exist when Kievan Rus' existed. Instead all three (Russia, Ukraine, Belarus) inherited the ethnicity of Kievan Rus', and then changed over time into the ethnic divides we see today.
Now to throw a little wrench in the machine though. According to the Primary Chronicle and is generally accepted story is that the rulers of Kievan Rus' were originally not Slavic and instead were Nordic and the descendants of the non-Slavic family would rule over Kievan Rus', and eventually over Muscovy (which led to Russia).
You may be interested in some of the previous posts on Kievan Rus' wrt modern ethnicities
Background
I have heard that the Ruric dynasty of Kievan Rus were vikings, how true is that? - /u/superplaner introduces various theories about the identity of the Rus'. To illustrate, this thread includes several conflicting views Was Kievan Rus' founded by Vikings?
Was Kiev the first "Russian" city? Or did the Russian state not exist yet? - /u/TerribleTauTG and /u/Cerebusial expand on early Kiev
Russia during the Keivan Rus period - /u/haimgelf, /u/monster_shoe, and /u/Omnipotent_Kanye discuss Kievan Rus' political structure
Development
When did Russian culture become a thing? - /u/rusoved and /u/azdac7 explore the beginnings of "Russian"
How did Belorussians and Ukrainians evolve as distinct national identities from Russia? - /u/cheapwowgold4u gives an overview of the formations of Ukrainian, Belorussian, and Russian
When did Ukraine become culturally separate from Russia? - /u/rusoved and - /u/kaisermatias take a deeper dive into the formations of the various ethnicities
How did the ukrainian culture develop after the Kiever Rus? Why did people claim ( and still do ) that the ukrainian culture and ethnicity are made-up/fictional? /u/grayraw focusses on early Ukrainian history
These posts have all been archived by now, so if you have follow-up questions for any of the users, just ask them here & mention their username to notify them