Pip describes what he looks like in direct characterization in chapter 1.
In the beginning of the book, Pip describes himself as a child. When Magwitch says that he has fat cheeks, he thinks about himself.
I believe they were fat, though I was at that time undersized, for my years, and not strong. (ch 1)
Pip is pretty honest about who he is in the beginning. He knows he is a little, scrawny, timid kid. Besides physical descriptions that are direct characterization like this one, there is plenty of indirect characterization. Besides being afraid of Magwitch, Pip also pities him. He does not just help him because he is afraid, he also does it because he wants to make sure the man gets some food.