Cindy-Lou Who only appears once in the story, so we may be tempted to just write her off as nothing more than a cameo. As in, oh, she's just a little tot that the Grinch ran into while he was on his mission to destroy Christmas! No big dealio.
Not so, Shmoopers, not so.
In the book, she's depicted as a tiny, innocent little thing with wide eyes and a sweet pink nightgown. She's obviously quite the opposite of the Grinch, and when she approaches him, she's not out to get him. She's not even angry or mean.
She truly wants to know why he is taking away all their Christmas things, and when he makes an excuse, she is perfectly satisfied. Oh, of course you're repairing our Christmas tree. Her innocence and gullibility protects her from suspecting the Grinch of anything nefarious, even though we can't help but wonder why she doesn't think it's odd that there's a giant, fuzzy cave-dweller in her living room. Huh.
The thing is, Cindy-Lou serves as a kind of trigger to the Grinch's change of heart. True, when he runs into her, he doesn't immediately throw up his hands and proclaim, "I've changed my mind! Christmas is a wonderful holiday after all!"
But still. It's the first time we see the Grinch do something un-Grinchy and actually behave nicely towards anyone. He doesn't simply shove Cindy-Lou aside and escape up the chimney. He kindly makes her a hot drink and sends her off to bed.
And his fib fooled the child. Then he patted her head
And he got her a drink and he sent her to bed. (114-115)
As he sweetly addresses Cindy-Lou, the Grinch appears almost benevolent, all his frown lines gone. Instead, he's all pleasant smiles and full of "my dears," showing the reader—for the very first time—that the Grinch could be a pretty nice guy if he tried.
True, he also lies to her about why he's taking the Christmas tree, but this is a decided improvement to Mr. Grinch's usual demeanor. And why lie in the first place, if it's not to protect this little chickabiddy's feelings?
You could say that Cindy-Lou is not so much a character as she's the thing in the story that gets the ball rolling. We can't prove it beyond a shadow of a doubt, but we think that the encounter with Cindy-Lou is what sets the Grinch's mind working. He begins to think that maybe these Whos aren't so bad after all. Maybe they're just nice folks who want to enjoy their holidays.
In the end, it's the Cindy-Lous of the world, with their bright spirits and kind hearts, who win out over darkness and Grinchyness.