My original guess would've been Rainbow Man, going by the stage. I guess that's why Capcom had the robot master names appear first thing before the stage.
Excluding the Cossack stages ones, this is the most russian-liked theme out of the eight robot masters. And yes, the first time I played this was the robot master I hated the most. Both stages and boss fight. (This was the first stage I entered and perhaps it wasn't a good idea... at all.) However, by replaying my grudge against him is gone. Now is focused to the first Cossack Stage.Icy floor, the robots which come out from the pitfalls without advicing, and the motherfucking badass russian theme which induces me that this game was the USSR revenge against the world... Awesome pal, keep it up!!