How is lennie described?

When we first meet Lennie, he is travelling with George to their ranch job. Lennie is easily characterized as slow or mentally challenged. He is a large adult male with the emotional literacy of a six year old. Lennie can't remember where they are going and George needs to remind him, "I gotta tell you again...Jesus Christ you’re a crazy bastard!" Lennie does childish things like hold onto a mouse. There is still innocence about him even though he is in the body of a strong man.