The Kievan Rus was in fact almost Catholic since it started as a Viking kingdom by king Oleg, son of Rurik
A little bit of a nitpicking

cool

: AFAIK, Oleg's relation to Rurik is anything but clear and most probably he was NOT his son and perhaps not even a blood relative. Which, of course, does not prevent his descendants from considering themselves Rurikids.
who had conquered or at least settled an area around northwest Russia and Ukraine.
Oleg hardly "settled" the area. The "Rurikids" had been moving Southward from Novgorod (a reasonably big city at the time of Rurik). By the time of Oleg's arrival, Kiev already existed (and was ruling by other Varangians, at least name of one of them, Askold, looks Nordic). There were principalities out of the Dnieper route (which was only one of the major trade routes), like Polotsk on Dvina River, or Chernigov on Desna, some of them ruled by non-Rurikid dynasties. The Scandinavians had been forming a ruling/military class but they were not numerous, hence their fast "merge" with the local culture: the 4th Rurikid ruler already had a Slavic name, Svyatoslav.
This was around the 10th century when the Vikings hadn't fully converted yet to Catholicism, but by that time Catholicism was already making a lot of progress. It's quite possible that Oleg or one of his successors could have converted to Catholicism.
Of course (even putting aside the fact that he died in 912, the very beginning of the X century

). Close relations with the Scandinavian countries had been maintained for the few generations to follow, especially with Norway. Christianization of Norway started only in 1000, well after it happened in Rus, but it happened much earlier in Denmark and Sweden so Catholicism could be picked from these areas. Poland is more problematic: official Christianization started only couple decades prior to Vladimir's baptism of Rus.