In chapter 1 we get a very detailed description of who Dill is, and we get a vivid description of his imagination and his personality.
"Dill 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 [Scout's] junior but [she] towered over him."
He had blue eyes that danced when he told his stories, he laughed a lot, and he constantly pulled at a cowlick that was in the center of his forehead. When asked personal questions, such as where his father was, he instantly blushed.
There a few other things we can say about Dill.
First, he is short. This is probably the first detail. In a conversation with Jem, Jem points out this fact.
“Shoot no wonder, then,” said Jem, jerking his thumb at me. “Scout yonder’s been readin' ever since she was born, and she ain’t even started to school yet. You look right puny for goin’ on seven.”
Second, he looks very young, probably because of his short stature. Jem asks how old he is, guessing four.
“How old are you,” asked Jem, “four-and-a-half?”
Third, he has a cowlick in the center of his forehead. Scout states:
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.
If you put it all together, one gets a sense that Dill is a small, smart, sickly, and highly eccentric child.