Whether we put a Russian character in the game or not will ultimately boil down to how many more characters we can squeeze in, given our available resources.
Ignoring Oscar, Pedro and Nikos, at the very least the game will have three more recruitable characters, all optional. Without these characters the game is technically incomplete. Of those three, none are Russian. However, we're leaving room in the game for up to 4 more optional characters in case we have the fuel to put more in. If we do put them in chances are one will be Russian and the other from the Pacific.
If we can't add the 4, though, chances are we will reuse some of our ideas for them to make non-recruitable mythicals that can show up in other parts of the story. For instance, we are playing around with the idea of Asterion's therapist (which won't be recruitable, he just plays a role in Asterion's self-improvement) being from Egypt.