In Harper Lee's To Kill a Mockingbird, the reader is presented with a description of Dill in Chapter One, which comes from conversation between Jem and Dill when the children first meet, and through observations in the voice of Scout, the book's narrator.
One day the children hear something in Miss Rachel's garden and go to investigate—Miss Rachel's dog is pregnant and they think that perhaps it is a puppy. Not an animal at all...
...instead we found someone sitting looking at us. Sitting down, he wasn't much higher than the collards.
Dill introduces himself as Charles Baker Harris. Jem, not one to welcome a new friendship too easily, is agressive and asks this "stranger" how old he is—the insult comes from his question/answer in terms of years: "four-and-a-half?" The discussion is based on Dill's announcement that he can read, and when Jem finds out that Dill is seven, he starts "slinging a tall tale," specifically that it's not a surprise to him that Dill should read at seven because...
Scout yonder's been reading' ever since she was born...
You look right puny goin' on seven.
Dill is not put off:
I'm little but I'm old.
This is a fair statement for Dill is something of an "old soul." Jem continues to "test" Dill, telling him that his name is longer than he is, but Dill stands up for himself, saying that Jem's name isn't much better ("Jeremy Atticus Finch"), and then he gives the Finch children his nickname: Dill.
Dill comes under the fench into their yard and he begins to share some of his background: where he's from, details about his family, but most importantly for Jem, that he's been to the "picture show" (movies) often. This is a novelty in Maycomb where the only movies that have ever been shown to Jem's knowledge are the "Jesus ones." The deal is sealed when Dill admits that he has seen Dracula.
Dill had seen Dracula, a revelation that moved Jem to eye him with the beginning of respect.
As Dill tells them the movie version of the vampire story, Scout studies the young boy:
Dill was a curiosity. He wore blue linen shorts that buttoned to his shirt, his hair was snow white and stuck to his head like duckfluff; he was a year my senior but I towered over him. As he told us the old tale his blue eyes would lighten and darken; his laugh was sudden and happy; he habitually pulled at a cowlick in the center of his forehead.
Soon, Jem's tells Scout to stop pestering Dill for information:
...a sure sign that Dill had been studied and found acceptable.
(Research notes that the character of Dill was fashioned after the idiosyncratic writer Truman Capote, who was a childhood and, later, lifelong friend of Harper Lee.)*
Perhaps the best thing about Dill is his wonderful imagination, to which Jem is immediately drawn. Whereas things had been boring and repetitive with Scout always playing second to Jem's leading roles, Dill provides a new depth to their vivid role-playing. It is Dill who becomes obsessed to learn more about Boo Radley: to get him to come out so they can see him, talk with him, and learn more about him.
It is safe to say that with Dill Harris, good things do come in small packages!