Snow is a cyberneticist, and he functions as something of a foil for Kelvin. When Kelvin wants to take Rheya to earth, Snow explains why he can't and that he's stupid. When Kelvin is sick of the whole idea of alien contact, Snow says contact is fun, now more than ever. We only catch passing glimpses of Snow's visitor, and similarly we only really get glimpses of Snow himself. He pops up enough that you feel like you should know him better, which must be the way all those Solarists feel about Solaris after studying it their whole lives.
Another scientist, Sartorius mostly stays locked in his room with his visitor, who may or may not be a child. Sartorius is officious and unpleasant, so maybe it's just as well he stays out of our way.
Gibarian was Kelvin's instructor at college, but he kills himself just before Kelvin arrives at the station. Everybody around Kelvin seems to commit suicide. Huh.