Kris Kelvin is defined by his past. Except he doesn't seem to have much of a past. And yes, that means he doesn't have much definition. Which is odd for a central character, and leaves Solaris unusually empty, not unlike that experiment in the novel where Kelvin looks at Rheya's blood under the microscope closer… and closer… and then realizes that, at the sub-molecular level, it doesn't exist.
Of course, Rheya is a construct made by a big sentient ocean, but Kelvin is a real person. So while it makes sense (in the land of science fiction, anyway) that Rheya sort of fails to exist when examined too closely, the vagueness that surrounds Kelvin as our main character defies the paradigm we're used to encountering when it comes to protagonists.
For instance, what do we know about Kelvin and his life with Rheya? She's obviously the most important person in his life—she's what the ocean pulls out of his subconscious, after all—but he doesn't actually seem to think about her. We're supposed to be in his head throughout the book, but the glimpses of their relationship in the past are scattered and barely articulated.
In fact, at one point he tells Rheya that he doesn't even remember the original Rheya. "Now all I see is you" (9.186), he tells her, which is sort of sweet, but also kind of creepy. You could see it as meaning that Kelvin's memories have totally replaced the real Rheya, but you could also see it as meaning that Kelvin has no memories; he's just this voice on the page, for whom nothing has ever happened before. Rheya isn't a memory; she's just a fiction. There was no real Rheya, and since Rheya is Kelvin's past, that means there's no real Kelvin either.
It's not just in his dealings with Rheya that Kelvin seems vague. A lot of his time in the book is spent summarizing his reading about Solaris and Solarists. Sometimes he's looking for a particular reference, sometimes he's deliberately distracting himself, but in every case, the focus isn't on him as a character and we find ourselves wondering who, exactly, he's talking to.
Usually when you read first person narration, it's convention to assume that you're hearing the person's thoughts, pretty much, telling you what they're doing. But having a character summarize a bunch of reading starts to beg the question. Especially when, as here, the reading is dry, often pointless, and rehashes things Kelvin has already read. When he tells you the history of Solaris studies, then, it seems like he's doing it mostly because Lem wants us to know the history of Solaris studies; Kelvin is just a convenient fiction.
Which, again, makes it seem like Kelvin isn't really there. He's somebody Lem made up, and set walking around and babbling about the history of Solaris. And while characters are pretty much always made up, Kelvin's fictitious nature is highlighted so much that the distinction between him as a person or just a word gets really blurred.
"My mind was a blank" (10.124): So Kelvin says toward the end of the book, and it seems like more than just a passing observation. His mind generally seems to be a blank; the place where his past should be is empty or obviously artificial.
At one point, he expresses nostalgia, not for his former life on Earth, but for the military drill of the ship he was on previously—"For a fraction of a second, my throat tightened and I thought longingly of the Prometheus and its strict discipline" (1.67)—as if his memory doesn't extend beyond this. In another passage, he says he knows the Solaris station because he trained on an "exact replica"(4.1) of it; his memories, then, are replicas, not the real things. So what is he?
When Kelvin has dreams that "my body is clothed in a dead, formless substance—or rather, I have no body, I am that alien matter" (12.10), it's a good description of how he appears to the reader throughout the book: empty, other, there but not there.
Kelvin also says, "Apathy robbed me of the strength even to despise myself" (10.124). That seems like a general truth about him, too; for a main character, he does little, and that little is almost entirely ineffectual. It's not him, after all, who gets rid of the visitors; it's Snow and Sartorius. Kelvin tries to stop them, but his hapless flailing in that direction is useless.
He also talks vaguely to Rheya about getting a dream house on Earth, but doesn't do anything about it, or about anything, except for locking himself in the library and reading books he already has read about how no one knows anything about Solaris. As science fiction heroic protagonists go, he's really more a wet dishrag of a nonentity than, well, a science fiction heroic protagonist.
You could see Kelvin's blah-ness and unreality as a function of the fact that he's a character in a novel. Solaris is all about artificiality and fakeness, so it makes sense that the man at the center of it should be fake as well. A book aware of its own constructed nature should have a narrator who is a construct. Kelvin is like that artificial garden in the ocean: He's obviously fake.
It's also possible, though, to see Kelvin's emptiness as due to the fact that he's made of grief. Kelvin has no past because the past is Rheya, and he doesn't want to think of Rheya; he has no present because you can't exist without a past. Rheya is more real than him, because she's the only real thing about him. He's been hollowed out—"I hoped for nothing. And yet I lived in expectation. Since she had gone, that was all that remained" (14. 69)—and since he's defined by his past, and he has no past, he's a void.
From this angle, Solaris is about people who are not there, both the mourned and the mourning. And since no one knows much of anything about the planet Solaris, grief emerges as the only certainty in this book. Deep, right? Oceanic, you might even say.