What is the importance of Crooks' character in Of Mice and Men?Please, can you include some quotes as well? I'm really struggling with of mice and...

In Of Mice and Men, Crooks is used to
juxtapose Lennie as the two "weak ones" left behind by the men when they go to the
cathouse.  As such, Crooks and Lennie are the lowest
ranking men in the Social Darwinian hierarchy of the
ranch.

Whereas Lennie the mentally weakest of the men,
Crooks is the strongest academically.  He is surrounded by
books and lives the life of an academic in his spare time.  Crooks repeatedly calls
Lennie "crazy."

Whereas Lennie is the strongest of the men,
Crooks is among the weakest (he's probably stronger than Candy).  As such, he feels
threatened by Lennie.

Crooks, then, is the lowest member on
the ranch: lower than Lennie, Curley's wife, and even Candy.  At least these characters
have partners: Crooks is completely alienated.  As such, he is an
outcast.  Observe:

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"S'pose you didn't have nobody. S'pose you
couldn't go into the bunk house and play rummy 'cause you was black. How'd you like
that? S'pose you had to sit out here an' read books. Sure you could play horseshoes till
it got dark, but then you got to read books. Books ain't no good. A guy needs
somebody-to be near him. A guy goes nuts if he ain't got nobody. Don't make no
difference who the guy is, long's he's with you. I tell ya, I tell ya a guy gets too
lonely an' he gets sick"
(80).

Whereas
Lennie is the keeper of the dream, Crooks is the realist, killjoy, prig,
straight man: the one who sees through the dream.  He knows the false
idealism of the Dream Ranch:

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"I seen hunderds of men come by on the road an'
on the ranches, with their bindles on their back an' that same damn thing in their
heads. Hunderds of them. They come, an' they quit an' go on; an' every damn one of 'em's
got a little piece of land in his head. An' never a God damn one of 'em ever gets
it. Just like heaven. Everybody wants a little piece of lan'. I read plenty of books out
here. Nobody never gets to heaven, and nobody gets no land. It's just in their head.
They're all the time talkin' about it, but it's jus' in their head"
(81).

Whereas Lennie is an
alazon (thinks he's better able than he really is), Crooks is an eiron,
(one who is capable of more than he has
done).

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Maybe you guys better go. I ain't sure I want you
in here no more. A colored man got to have some rights even if he don't like 'em"
(90).