Role in the adaptations
In the original book and the 1966 animated adaptation, two-year-old Cindy Lou Who gets out of bed on Christmas Eve to get a drink of water when she finds the Grinch, disguised as Santa Claus, stuffing her family's Christmas tree up the chimney. Mistaking him for Santa, she innocently asks him why he's taking the tree. Sensing innocence in the young girl, the Grinch covers up his theft by saying that there is a broken light on the tree, and that he is taking it to "his workshop" to fix the problem and then return it on Christmas Day when she wakes up. The Grinch then gets her drink and sends her back to bed, showing the only legitimate kindness he has in his tiny heart. Although this is her only real scene in the original book, Cindy Lou has a bit more screen time in the animated adaptation, such as appearing during the opening song and later giving Max a slice of the roast beast as the Grinch joins the Whos for Christmas dinner at the end.
In the 2000 live-action version, Cindy Lou is about six years old. Her father, Lou, who was never mentioned in the book or the TV special, is the Whoville Postmaster, or head mailman, who occasionally delivers the papers as well. Unlike in the book and TV special, Cindy Lou first meets the Grinch early on in the story when he inexplicably saves her from a stamp machine in the Whoville Post Office after initially startling and scaring her, causing her to fall into the chute leading to the stamper. This follows after being told of the Grinch's existence by her two older brothers Stu and Drew after they were scared off of Mount Crumpit by Max after finding the Grinch's front door, and getting in trouble with her teacher Miss Rue Who for mentioning the Grinch in class, forcing her to stay after so Miss Rue could speak to Lou on the matter. After doing some investigative reporting about him, including interviewing Mayor Augustus May Who and Martha May Whovier, Cindy is the only one in Whoville (aside from Martha, the Grinch's childhood crush) who believes that the Grinch is simply just a misunderstood creature, and suggests that they let the Grinch be the guest of honor in Whoville's holiday party, the 1,000th Whobilaiton, as the Holiday Cheermeister. Although that idea ultimately turns sour thanks to Mayor May Who's cruelty, resulting in the Grinch causing chaos and trying to ruin the Whobilation before returning to Mount Crumpit, Cindy still believes there to be some good in the Grinch; when the above-detailed scene from the book comes up, she tells "Santa" that he should not forget the Grinch. The next morning, Augustus angrily accuses her of robbing everyone when the Whos awake to their homes being ransacked by the Grinch overnight, but her name is cleared shortly after her father stands up for her and tells off the Mayor, realizing Cindy was right that family is what's important on Christmas, not the glamour and gifts, though the Mayor still refuses to accept it. She then helps the Grinch return every item back to the Whos after sneaking aboard his sleigh by telling him that no one should be alone on Christmas, resulting in the Grinch saving her from plunging to her death with the sleigh when he sees her in danger. When they return to Whoville, the Grinch surrenders to Police Chief Wholihan, but Wholihan grants him a full pardon for returning everything and apologizing, and Martha calls off her engagement with Mayor May Who, as her heart truly belongs with the Grinch, leaving May Who to suffer a humiliating defeat at having lost Martha and Whoville to the Grinch and Cindy. Cindy and the Lou Whos later join Martha and the Grinch in celebrating Christmas in the Grinch's cave on Mount Crumpit with the rest of Whoville, with the Grinch having the honor of carving the roast beast with Martha by his side as Cindy and her family sit nearby with Max.
The 2018 CGI film's version of Cindy Lou Who differs further from her previous incarnations, one major difference from the previous film being that she lives with an overworked widowed mother. Like in the live-action version, Cindy Lou encounters the Grinch early on in the story, in this case when she's going out to mail her letter to Santa Claus, requesting that he help her mother with her workload. The Grinch sarcastically remarks that she should talk to Santa in person, which begins Cindy's own subplot in the movie. With help from her friends, Cindy Lou puts together a plan to catch Santa when he comes to her house on Christmas Eve. This of course leads to the scene from the book where she encounters the disguised Grinch, who has fallen into her trap. Her request for "Santa" to help lighten her mother's workload touches the Grinch's heart and leads him to reconsider his actions. After the reformed Grinch returns everything he stole, Cindy Lou invites him to her family's Christmas dinner.