Lennie is a huge, grown man - but also very childlike
Lennie is powerful with huge hands - this makes him a good ranch worker
Lennie's grown up physically, but mentally he's still like a child - "He's jes' like a kid"
Lennie's innocent and asks a lot of questions. Slim immediately sees that Lennie "ain't mean"
Lennie's condition is never explained. He's called a "dum dum" by Curley's wife and Slim thinks he's "cuckoo", but George denies that he's insane.
Lennie likes to stroke and "pet" soft things like mice and Curley's wife's hair. He's like a child with a stuffed toy.
Lennie is describes like an animal - "Lennie covered his face with his huge paws and bleated with terror"
Lennie is described like a bear - he drags his feet "the way a bear drags his paws"
Lennie is very possessive over his animals, he never wants to let them out of his site. He's like a child with a favourite toy.
Lennie is a bit like George's pet. He follows George around and relies on him for food. He also obeys George - at the pool he brings George the mouse "like a terrier who doesn't want to bring a ball to its master."
George treats Lennie like a pet too - he orders him around and uses his strength to get them jobs. In the end, he treats Lennie in the same way that Candy treats his dog - he shoots him for his own good.