Russia would probably extend more influence over Korea, including perhaps the establishment of a few Eastern Orthodox churches or so on. I am thinking this does leave Japan without Korea and while they may wanna try again, whether they could is a different matter entirely. It would also affect Japan's standing on the world and how they are viewed. Korea would probably be grateful, especiall if Russia uses the chance to rengeotiate some of the unfair treaties Japan forced on Korea.
This could mean that Japan stays out of the Great War though it's a toss-up. They may not be taken seriously though. I am thinking when Russia begins undergoing its breakdown though it may first try to get the Sakhalin island once more as a way to get that first over Korea. At the same time, Korea becoming a warm water port in the Pacific for Russia may mean further build up and reinforcements along with a bit more time to modernize there. I am thinking Japan's intents would fail after the Red Army beats them there and frustrates them further.
Korea meanwhile would likely be trying to finish modernize or be concern with what is going on with Russia and China and try to move closer to the American sphere or come up with an arrangement with the Soviets though it would also depend on the Korea socialists. I am thinking the USSR doesn't worry too much on it and expects Korea to be loyal though the socialists will get some support. I doubt Kim or the others however.
The big issue is if Japan gets a neutrality pact with the USSR, since I imagine it would extend to Korea. If it does, then Korea gets mroe time to prepare and likely communicates a bit with the Americans as part of the Allies during WW2. Japan going full axis will be interesting since without Korea, they will have a harder time especially a foothold into China, but This means Japan has to have a bit of a harder time getting a foothold into China and cannot predict how that attempt will go. Eventually, Korea will join alongside the USSR in war against the Japanese. Korea will likely help out the Chinese and probably focus on Manchuria or whereever the Japanese set their foothold while being defensive. they may try though Taiwan or so on, but Korea will likely be occupying more and more of the area and fighting alongside Americans.
They might still get Manchuria occupied with Soviet help though they might decide who they pass it to. Either the Communists or the Kuomintang; I suspect who gives them the better deal in terms of leaving Korea alone and maybe a bit of land. If it's the Communists, it will be just like usual though with the promise of leaving Korea along by China and perhaps the USSR though they probably won't trust them. Perhaps they might give it to the Kuomintang with the hopes a Kuomintang China will be at odds with the USSR.
Korea might end up having some rebuilding by the Americans, the American sreasoning it would keep them out of the Soviet sphere if they weren't. If China is nationalist, Korea will feel a bit more ease though probably remain in the American sphere for safety against them ans have positive memories of them because of the war. If China is communist, does also put them in a tense spot and may still result in them having a red leader forced upon them so China can have a meatshield.