Who is Orlick and what does Pip think he did?
Pip is given clues about how Mrs. Joe became injured. We do find it was Orlick, but Pip thinks it was him for about 30 chapters before it is verified.
One day while Biddy and Pip are talking, Biddy remembers some of the things that Mrs. Joe has tried to communicate. Mrs. Joe kept writing this character on her board that looked like a t or a J. Thinking that the J had something to do with Joe, the idea of Orlick was regularly dismissed. But when Biddy and Pip put together that the t or J might have been a hammer, everything begins to make sense. The hammer represents Orlick because that's what he does as a blacksmith.
Orlick is one of the worst characters in this book. He is a big man who works for Joe Gargery at his forge. He really hates Pip. The reason for this is that he feels that Pip is Joe's favorite.
Later on in the book, Pip thinks that Orlick is the one who hit Mrs. Joe in the head. This results in her being mentally incapacitated and eventually kills her.
As it turns out, Pip is right to think that Orlick did that. Orlick admits it to Pip in Chapter 53. He says that he found the leg iron that Pip had filed off the convict. He held on to it until he used it to hit Mrs. Joe.