Lennie

Lennie, the protagonist's trusty sidekick in Of Mice and Men, is arguably more significant to the book than George.
At the beginning of the book, one of the first major points made is that Lennie does not fathom his own strength. This oblivion springs much of the plot of the book into action. At first, Lennie only harms mice when he pets them. George tells Lennie he can get a dog, thinking that a larger animal will allow for less of a chance of harm to the creature. Lennie gets a dog to call his own, and loves it and pets it. Eventually, the little puppy bites Lennie though, and he smacks it. Needless to say, the puppy died. He is distraught over killing the puppy, and begins to blame the puppy in a way, asking it why it died, yelling "you ain't so little as mice!"
Lennie begins to hurt humans as well. George tells Lennie not to fight with Curley, but Curley starts a fight and Lennie unwillingly defends himself, breaking Curley's hand in the process. George had also told Lennie to stay away from Curley's wife, but sure enough, she cajoles him into talking with her. Lennie tells her he just likes to pet soft things, and she lets him feel her hair. When she tells him to let go, he panics, holds on tighter, she screams, he shakes her, her neck breaks, and she dies. Lennie never means to hurt anyone or anything and he has some of the best intentions of any character, but he often inadvertently harms those he loves, along with those he doesn't care for.
Although Lennie may not be very bright, he has a heck of an intuition. This book is filled with foreshadowing, and one of the first examples of foreshadowing comes from Lennie. He tells George almost immediately after they arrive at their destination that he doesn't like it there, and thinks they should get out as soon as possible. George, of course, disregards Lennie's pleas and tells him they must stay to make some money. Little does he know this place will be the death of Lennie.
Lastly, even though George is clearly in place as Lennie's caregiver, Lennie also feels quite the responsibility of looking after George. George is always advising Lennie and looking out for him, and doing his best to keep Lennie out of trouble. When George leaves to go to a whore-house with some other guys, it brings out a turning point in this relationship. Crooks begins talking to Lennie, telling him that there is a possibility that George isn't coming back and that he might have gotten hurt. Crooks is just randomly saying this, of course, but Lennie is nervously hanging on to his every word. He believes Crooks and wants to find out who hurt George. Lennie becomes very threatening toward Crooks and Crooks realizes he should back down and reassure Lennie that George is okay. Lennie obviously feels a sense of protection for George. Maybe it's just because he knows he needs George, but I think Lennie genuinely worries about George with little regard for himself.