Lennie Small is huge and lumbering and, in many ways, the opposite of George Milton. Where George has sharp features and definite lines, Lennie is "shapeless." Often he is described in terms of animals. He lumbers like a bear and has the strength of a bear, but his actions are often described like those of a dog.
Lennie's personality is like that of a child. He is innocent and mentally handicapped with no ability to understand abstract concepts like death. While he acts with great loyalty to George, he has no comprehension of the idea of "loyalty." For that reason, he often does not mean to do the things that get him into trouble, and once he does get into trouble, he has no conscience to define his actions in terms of guilt. Lennie only defines them in terms of consequences: "George is going to give me hell" or "George won't let me tend the rabbits." He is devoted to George like a dog is devoted to its master, and he tries to follow George's commands. There is a childlike wonder in Lennie that can be seen when he first sees the pool of water and slurps down huge gulps of water like a horse.
Lennie's greatest feeling of security comes from petting soft things. When the rest of the world gets complicated and scary, petting soft things helps Lennie feel safe. In petting dead mice, Lennie is doing something that makes him feel safe. Society as a whole would disapprove of what he is doing, but Lennie sees nothing wrong in his actions. When they have their farm, as George tells him at the end, Lennie will not need to be scared of bad things any more, and he can tend the rabbits and pet them.
Lennie's prodigious strength combined with his lack of intelligence and conscience make him dangerous, and he needs George to keep him out of trouble. George takes care of Lennie and makes the decisions for him. George also gives him advice and helps Lennie when overwhelming forces, like Curley, scare him. George keeps the dream out in front of the huge man as a goal: Their farm is a place where they can live together, have animals, grow their own crops and, in general, feel safe. Lennie has little memory, but the story of their dream is one he knows by heart. While George never really believes in this farm, Lennie embraces it with childlike enthusiasm. Every time he makes George tell their story, his enthusiasm excites George, too. Lennie's innocence keeps the dream alive, but his human imperfection makes the dream impossible to realize.