In the initial descriptions of Lennie and George, the narrator notes that they are dressed the same, but this is where their similarities end. In many ways, they are opposites. George is small, wiry, smart, and a leader. Lennie is large, simple-minded, passive (unless he begins to panic), and a follower. Note also that in the fourth paragraph, they walk single file: George in the lead, Lennie following. Regarding their appearances, the narrator/Steinbeck says: 

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. 

Lennie is also described as "snorting into the water like a horse" when he goes to drink from the pool. The physical descriptions of both Lennie and George fit their personalities well. George is small and strong. He is quick, mentally and physically, and his restless eyes indicate that he is always looking out for himself and Lennie. Lennie's personality matches his physicality as well. He is large and timid; his timidity is shown by his sloping shoulders, his lack of confidence. In these opening descriptions, Lennie is described as an animal (like a bear and a horse). There is nothing overt here in terms of implying Lennie has the mind of a child. However, describing him as one who is large, slouched, and resembling/acting like an animal, the reader does get an initial impression that Lennie is unaware of how his behavior appears to others. The reader certainly gets an impression that Lennie is the opposite of the smaller, quicker man whose eyes were "sharp." Simply by opposing the two, the reader can suppose that Lennie is not as "sharp" as George. 

In the first chapter. the reader gets the picture of two men, one of whom is small, but the leader of the two men, and the other is big, but mentally impaired. Riding on the bus, George keeps complaining about Lennie's behavior, but Lennie responds with childlike answers. Lennie's obsession with the mouse and the way George explains why he can't have a dead mouse are indications that Lennie is not very intelligent. Then, when Lennie responds to George's outburst about not having ketchup for the beans, Lennie responds like a small child and says he can just live in a cave. Steinbeck also describe Lennie's hands as paws when he drinks from a pool that may not contain fresh water. George has to remind him that all water is not safe. So there is no need for Steinbeck to tell us directly about Lennie's mind or the relationship between Lennie and George. The author shows it to us through their actions and dialogue.

With the first descriptions of George Milton and Lennie Small, Steinbeck establishes the relationship of the two characters, as well as some of their individual characteristics.

Certainly, the names of the two men are suggestive of their personalities. Named after English kings and a renowned English poet, George Milton leads Lennie Small into the clearing. He is "small and quick...with restless eyes...strong features. Every part of him was defined." This description suggests that George is intelligent and very aware of his surroundings and fit to deal with things. The large, bear-like man follows in a subservient posture with

...with wide sloping shoulders...[as] he walked heavily, dragging his feet a little, the way a bear drags his paws.

Lennie imitates the actions of George, and asks questions of George as a child would. That his last name Small is suggestive of a deficiency, which is revealed to be a mental one as he is told what to do by George and scolded for having a mouse in his pocket as a boy would do. George's words, "I could get along so easy and so nice if I didn't have you on my tail" indicate that Lennie is a burden to George.

And, yet, George does have feeling for Lennie, the keeper of their "dream." As he recites, George's voice deepens. He repeats with a certain rhythm:

Guys like us...got no family. They don't belong no place. They ain't got nothing to look ahead to....With us it ain't like that....We got somebody to talk to that gives a damn about us.... Someday--we're gonna get the jack together and we're gonna have a little house and a couple of acres an' a cow and some pigs....

After reciting their dream, George hopes that Lennie will keep from getting into trouble because he does care for the man. He tells Lennie to pull his bindle over by him, and sleep by him as his friend.

One of the first descriptions of Lennie and George is of them walking "in single file down the path, and even in the open one stayed behind the other." (Pg. 2) This is a significant description because it shows that one of the characters is deliberately following the other--like a child.
The more detailed description of George shows that he has sharp, strong features and "restless eyes." These somewhat simple descriptions give insight into George's character. His strong features suggest that he is also strong mentally, and his "restless eyes" suggest that he is watching over something--he is very observant.
The descriptions of Lennie also give clues as to what his character is like: "pale eyes" and "wide, sloping shoulders." It is said that he "walked heavily, dragging his feet a little"--like a child.
The other information that suggests that Lennie has the mind of a child is the way in which he "flung himself down and drank from the surface of the green pool." (Pg. 3)
It is at this moment that George reprimands Lennie for drinking the stagnant water, warning George of how he got sick the last time. It is obvious at this point that George watches out and takes care of Lennie because he is unable to take care of himself.