Edit: I should clarify the title... fighting against Finland. On the Soviet side.
I know that speakers of Finnish dialects from Finnish Karelia and eastern Karelians in Russia can more or less be plotted along a cultural/ethnic/linguistic continuum.
It got me thinking about what it would mean for a Russian Karelian, say, one from Petrazavodsk or Kostamuksha, to be pitted against the Finns as a Russian soldier-- when he may be able to understand some Finnish dialects (certainly ones right across the border), and find the culture familiar.
Are there records of this happening? Was there a conflict of loyalty here? Just how Russian did Karelians in Russia feel during this period (vs. feeling alligned with the Finns, or perhaps only with Finnish-Karelians).
Yes.
Russian Karelians were Soviet citizens and therefore liable for service in the Soviet military. In the early days of the Winter War, Finnish-speakers were even collected into a special unit - the "Finnish People's Army" formed of the "First Finnish Army Corps" - that was to be the army of the Terijoki puppet government. In practice, this "corps" consisted of Finnish-speakers, those with Karelian or Ingrian ancestry, and Russians, Ukrainians and others who looked enough like Finns. The Soviet plan was to have 80 000 men in this army, but even though they combed even their own prison camps (at this point, many of the Finnish Communist emigrants had been shot or imprisoned), they could only collect 22 594 men in total. These men were dressed in captured Polish uniforms and equipment, and were not to be used for combat but only for propaganda purposes - like for the victory parade in Helsinki.
However, as this was not happening as soon as expected, some elements of this army fought in the Winter War. It was demobilized after the war and its personnel formed into new Red Army units.
Besides this show army, there were Karelians and Ingrians, some of whom spoke Finnish, in regular Soviet units. I can't say much about their morale and don't know whether they were more or less likely to defect or surrender, but it should be kept in mind that Karelians didn't think of themselves as Finns. There were distinct cultural and linguistical differences, the Soviet system had been fairly successful even in rooting out the Finnish language, and the Karelians had their reasons not to trust Helsinki government that much in the first place: Finnish nationalists had tried to annex Eastern (Russian) Karelia during the Russian Civil War and were often seen as conquerors. (Although this relationship was complicated to say the least - Finns also aided the Karelians in their abortive rebellion against the Soviet government in the early 1920s.)
The Finnish occupation government during the Continuation War didn't exactly help matters that much, since particularly in the beginning it was manned by rabid nationalists who set out to "Finnish" the Karelians. Some went as far as to demand the rooting out of the Orthodox church, or "Russkie religion" as they called it. Gaffes like that and outright mistreatment (which wasn't the rule, but did happen) didn't endear the Finns to the Karelians that much, and although there were some who clearly welcomed Finnish "liberators", surprisingly few left with the retreating Finnish troops in 1944. This was perhaps for the best, since Finland was forced to repatriate practically all Soviet citizens who didn't manage to flee to Sweden or further west - and these repatriates were in practice shipped directly to prison camps or to execution.
From what I've read, including veteran's memoirs, is that Karelians were much like the rest of the Soviet soldiers: not really that interested in the fight but fighting bravely nevertheless.
The key source here is Leskinen & Juutilainen, Talvisodan pikkujättiläinen, WSOY 1999.