The opening pages of the novella offer a description of George and Lennie. Even though this description is physical, it does show what their personalities and characters are like. Let me give the quote and then make some observations.
The first man was small and quick, dark of face, with restless eyes and sharp, strong features. Every part of him was defined: small, strong hands, slender arms, a thin and bony nose. Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws. His arms did not swing at his sides, but hung loosely.
George is quick, sharp, and strong. He is the one who thinks, makes decisions, and charts a path for the future. He is also the one with the dream and the plan to get there. On the other hand, Lennie is a massive man, and that is pretty much it. He is not decisive, quick-witted, or agile in any way. He simply exists.
We can also say that George is wise in the ways of the world. He knows how things work and reads people well. Lennie, on the other hand, does not know any of these things. He is just a child. This comes out in many ways, such as his love for animals and his innocence. Here is what George says to Slim:
“Sure he’s jes’ like a kid. There ain’t no more harm in him than a kid neither, except he’s so strong. I bet he won’t come in here to sleep tonight. He’d sleep right alongside that box in the barn. Well—let ‘im. He ain’t doin’ no harm out there.”