To clarify - when the Soviets established North Korea, what made Kim their choice? Were other candidates considered?
Were other candidates considered? Yes. Kim il-Sung was actually not their first pick, and with good reason. When he returned to Korea in September 1945, he had been out of the country since he was a teenager and spoke Chinese better than Korean. When the Soviets tasked him with giving a victory speech during a rally in October, his speech had to be written for him by Soviet-Koreans and he was coached (with limited success) on pronunciation.
Later North Korean propaganda portrayed this speech as the magnificent culmination of a decades-long fight against Japanese imperialism, with hundreds of thousands of adoring citizens cheering on a triumphant Kim. The reality was that he was only one of the speakers that day (although the rally was technically in his honor), and the people in attendance only half-understood him. They weren't exactly sold on the guy that the Soviets were desperately trying to bill as the "liberator of the Korean people." If you're emerging from decades of humiliating control by a foreign power, ideally you want your liberator to be a heroic figure.
Kim? Not so much so. Kim was:
So the Soviet propaganda machine had to go into overdrive trying to turn all of this into a net positive ("He isn't dressed like shit for fun! He just can't afford a tailored suit! He's just like you!").
The interesting bit here is that the Koreans' first response to him was actually the correct one even though they had no way of knowing it at the time. While Kim had been part of the Northeast Anti-Japanese United Army in Manchuria, he certainly wasn't running it (the Chinese were), he was reasonably successful but not ultimately much more than a minor figure, and he'd sat out the entirety of World War II in a Soviet army camp near Khabarovsk in Siberia. Again, very different from how North Korean propaganda later portrayed it!
The original person to get the top job in Soviet-controlled Korea was a man named Cho Man-sik, who had led a Gandhi-inspired nonviolent resistance against the Japanese. The Soviets initially liked this ("Always nice to work with someone who won't shoot back") but found him annoying to control. Cho had a bad habit of bitching about how the Soviet occupation was being run, and about what he saw as patronizing and unnecessary plans for a foreign trusteeship to oversee a reunited Korea. The Soviets eventually got sick of this and Cho "disappeared" in January 1946, having been on the job for only four months. He was never seen or heard from again.
When the Soviets established North Korea, what made Kim their choice? The short and simple answer is that Kim was: a). a fantastic propaganda pick, and: b). seemed happy to take orders from Soviet leadership. During the high-profile fights over how to move forward politically on the Korean peninsula (reunify the Soviet- and American-led halves? Communist government? Democratic government? Who gets the most influence around here?), he was pretty adroit at playing to what the Soviets most wanted to hear. Having spent 5-6 years with the Red Army in Siberia doubtless helped. While he was never fluent in Russian, he was certainly there long enough to figure out how things worked and how the Soviets saw the Korean situation. Namely, the Soviets had little to gain from reunification because the American-controlled half that would eventually become South Korea had 2/3 of the peninsula's population, and on numbers alone would likely exert greater influence than the North. Stalin was not a huge fan of this possibility.
So Kim argued that reunification was not immediately desirable (the Soviets liked hearing this), that the Koreans had an opportunity to create a Soviet-style government in the North that they shouldn't pass up (the Soviets liked hearing this too), and that the Koreans owed their loyalty to the U.S.S.R. and that independence should be a careful and deliberate process (and the Soviets really liked hearing this). Some of this was probably genuine on his part; he'd been interested in communism for a long time, had instituted collectivist policies in Manchurian villages he'd billeted in during the 1930s, and probably did see the U.S.S.R. as an ideologically wholesome patron. The rest of it must have been a bitter pill for an ardent nationalist to swallow, but it was nothing that couldn't be fixed once he had control of the country later -- and the knowledge that NK's value as a buffer state meant the Soviets were just going to have to deal.