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 and foreshadowing his eventual change of 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, and she learns of the Grinch's existence early on 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. 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. 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 Holiday Cheermeister in Whoville's holiday party, the 1,000th Whobilaiton. 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, 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 Cindy of robbing everyone when the Whos awake to their homes being ransacked by the Grinch overnight. Fortunately, her name is cleared shortly after her father, Lou Lou Who, stands up for her and tells off the Mayor, realizing Cindy was right and that family is what's important on Christmas instead of 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. After Police Chief Wholihan gives the Grinch a full pardon, Cindy and her family later join the Grinch in celebrating Christmas in his cave on Mount Crumpit with the rest of Whoville.
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.