I know people like Geralt and I understand them, but I find them really hard to explain in words. You kinda have to learn to read them and they won't be mysterious at all. To be honest, I find such people rather simplistic and boring. I'm not talking of the archetype of "dark brooding guy" here, I'm being a lot more specific. Geralt's portrayal is more cultural than you might think, it's related to the cultural mindset of the devs. This is why I find it so hard to explain that even attempting to do it seems daunting. Ideally you need to get to know such people for yourself.I have to give you the frustrating version. I can't think of any one thing about him I don't understand. He's a very realistic character - very. As such, he's the kind of dark, brooding guy that holds his emotions in really well, and so he's hard to read, I guess.
I think that's the best way I could explain it.
But the basic idea is that instead of imagining the guy as an enigma, think this: he's simple, cares for booze and women, likes good food and vulgar songs, has some (limited) values, likes to kill things, believes himself to be strong and tough, believes himself to be smart, but is simple (repeated for emphasis). That will be close to the truth.
I could never understand from the books what ladies found in Geralt. At all.