Lennie Smalls is dull-witted and clumsy with his strength. He often appears to irritate or frighten other characters (he frustrates George, his companion and guardian, while he frightens Curley). These interactions reveal his slow-nature, naive mindset, and undetermined strength, which ultimately leads to his demise.
You can find examples of these traits by going to the passages about Lennie's handling of small animals (Chapter 1 and likely the last chapter when he accidentally kills the pup/Curley's wife)