Who is Dr. Sartorius from Solaris and what is their importance?

Dr. Sartorius is a Solarist working on Station Solaris. He is studying the planet Solaris. When Kelvin arrives at the research station, he learns that Sartorius has locked himself in his laboratory, refusing to come out. Kelvin can hear what sounds like children's feet running around inside the laboratory, and when he threatens to break down the door Sartorius agrees to come out. Whatever is in the lab starts banging against the door—desperately trying to get to Sartorius. This child-sized apparition must be Sartorius's "ghost"—a creature created out of his most guarded memory by the ocean of Solaris.